diff --git a/plugins/libkolab/lib/kolab_storage_folder_api.php b/plugins/libkolab/lib/kolab_storage_folder_api.php
index 7280389e..0b9091f2 100644
--- a/plugins/libkolab/lib/kolab_storage_folder_api.php
+++ b/plugins/libkolab/lib/kolab_storage_folder_api.php
@@ -1,351 +1,361 @@
 <?php
 
 /**
  * Abstract interface class for Kolab storage IMAP folder objects
  *
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  *
  * Copyright (C) 2014, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 abstract class kolab_storage_folder_api
 {
     /**
      * Folder identifier
      * @var string
      */
     public $id;
 
     /**
      * The folder name.
      * @var string
      */
     public $name;
 
     /**
      * The type of this folder.
      * @var string
      */
     public $type;
 
     /**
      * The subtype of this folder.
      * @var string
      */
     public $subtype;
 
     /**
      * Is this folder set to be the default for its type
      * @var boolean
      */
     public $default = false;
 
     /**
      * List of direct child folders
      * @var array
      */
     public $children = array();
     
     /**
      * Name of the parent folder
      * @var string
      */
     public $parent = '';
 
     protected $imap;
     protected $owner;
     protected $info;
     protected $idata;
     protected $namespace;
 
 
     /**
      * Private constructor
      */
     protected function __construct($name)
     {
       $this->name = $name;
       $this->id   = kolab_storage::folder_id($name);
       $this->imap = rcube::get_instance()->get_storage();
     }
 
 
     /**
      * Returns the owner of the folder.
      *
      * @param boolean  Return a fully qualified owner name (i.e. including domain for shared folders)
      * @return string  The owner of this folder.
      */
     public function get_owner($fully_qualified = false)
     {
         // return cached value
         if (isset($this->owner))
             return $this->owner;
 
         $info = $this->get_folder_info();
         $rcmail = rcube::get_instance();
 
         switch ($info['namespace']) {
         case 'personal':
             $this->owner = $rcmail->get_user_name();
             break;
 
         case 'shared':
             $this->owner = 'anonymous';
             break;
 
         default:
             list($prefix, $this->owner) = explode($this->imap->get_hierarchy_delimiter(), $info['name']);
             $fully_qualified = true;  // enforce email addresses (backwards compatibility)
             break;
         }
 
         if ($fully_qualified && strpos($this->owner, '@') === false) {
             // extract domain from current user name
             $domain = strstr($rcmail->get_user_name(), '@');
             // fall back to mail_domain config option
             if (empty($domain) && ($mdomain = $rcmail->config->mail_domain($this->imap->options['host']))) {
                 $domain = '@' . $mdomain;
             }
             $this->owner .= $domain;
         }
 
         return $this->owner;
     }
 
 
     /**
      * Getter for the name of the namespace to which the IMAP folder belongs
      *
      * @return string Name of the namespace (personal, other, shared)
      */
     public function get_namespace()
     {
         if (!isset($this->namespace))
             $this->namespace = $this->imap->folder_namespace($this->name);
         return $this->namespace;
     }
 
 
     /**
      * Get the display name value of this folder
      *
      * @return string Folder name
      */
     public function get_name()
     {
         return kolab_storage::object_name($this->name, $this->get_namespace());
     }
 
 
     /**
      * Getter for the top-end folder name (not the entire path)
      *
      * @return string Name of this folder
      */
     public function get_foldername()
     {
         $parts = explode('/', $this->name);
         return rcube_charset::convert(end($parts), 'UTF7-IMAP');
     }
 
     /**
      * Getter for parent folder path
      *
      * @return string Full path to parent folder
      */
     public function get_parent()
     {
         $path = explode('/', $this->name);
         array_pop($path);
 
         // don't list top-level namespace folder
         if (count($path) == 1 && in_array($this->get_namespace(), array('other', 'shared'))) {
             $path = array();
         }
 
         return join('/', $path);
     }
 
     /**
      * Getter for the Cyrus mailbox identifier corresponding to this folder
      * (e.g. user/john.doe/Calendar/Personal@example.org)
      *
      * @return string Mailbox ID
      */
     public function get_mailbox_id()
     {
         $info = $this->get_folder_info();
         $owner = $this->get_owner();
         list($user, $domain) = explode('@', $owner);
 
         switch ($info['namespace']) {
         case 'personal':
             return sprintf('user/%s/%s@%s', $user, $this->name, $domain);
 
         case 'shared':
             $ns = $this->imap->get_namespace('shared');
             $prefix = is_array($ns) ? $ns[0][0] : '';
             list(, $domain) = explode('@', rcube::get_instance()->get_user_name());
             return substr($this->name, strlen($prefix)) . '@' . $domain;
 
         default:
             $ns = $this->imap->get_namespace('other');
             $prefix = is_array($ns) ? $ns[0][0] : '';
             list($user, $folder) = explode($this->imap->get_hierarchy_delimiter(), substr($info['name'], strlen($prefix)), 2);
             if (strpos($user, '@')) {
                 list($user, $domain) = explode('@', $user);
             }
             return sprintf('user/%s/%s@%s', $user, $folder, $domain);
         }
     }
 
     /**
      * Get the color value stored in metadata
      *
      * @param string Default color value to return if not set
      * @return mixed Color value from IMAP metadata or $default is not set
      */
     public function get_color($default = null)
     {
         // color is defined in folder METADATA
         $metadata = $this->get_metadata(array(kolab_storage::COLOR_KEY_PRIVATE, kolab_storage::COLOR_KEY_SHARED));
         if (($color = $metadata[kolab_storage::COLOR_KEY_PRIVATE]) || ($color = $metadata[kolab_storage::COLOR_KEY_SHARED])) {
             return $color;
         }
 
         return $default;
     }
 
 
     /**
      * Returns IMAP metadata/annotations (GETMETADATA/GETANNOTATION)
      *
      * @param array List of metadata keys to read
      * @return array Metadata entry-value hash array on success, NULL on error
      */
     public function get_metadata($keys)
     {
         $metadata = rcube::get_instance()->get_storage()->get_metadata($this->name, (array)$keys);
         return $metadata[$this->name];
     }
 
 
     /**
      * Sets IMAP metadata/annotations (SETMETADATA/SETANNOTATION)
      *
      * @param array  $entries Entry-value array (use NULL value as NIL)
      * @return boolean True on success, False on failure
      */
     public function set_metadata($entries)
     {
         return $this->imap->set_metadata($this->name, $entries);
     }
 
 
     /**
      *
      */
     public function get_folder_info()
     {
         if (!isset($this->info))
             $this->info = $this->imap->folder_info($this->name);
 
         return $this->info;
     }
 
     /**
      * Make IMAP folder data available for this folder
      */
     public function get_imap_data()
     {
         if (!isset($this->idata))
             $this->idata = $this->imap->folder_data($this->name);
 
         return $this->idata;
     }
 
 
     /**
      * Get IMAP ACL information for this folder
      *
      * @return string  Permissions as string
      */
     public function get_myrights()
     {
         $rights = $this->info['rights'];
 
         if (!is_array($rights))
             $rights = $this->imap->my_rights($this->name);
 
         return join('', (array)$rights);
     }
 
+    /**
+     * Helper method to extract folder UID metadata
+     *
+     * @return string Folder's UID
+     */
+    public function get_uid()
+    {
+        // To be implemented by extending classes
+        return false;
+    }
 
     /**
      * Check activation status of this folder
      *
      * @return boolean True if enabled, false if not
      */
     public function is_active()
     {
         return kolab_storage::folder_is_active($this->name);
     }
 
     /**
      * Change activation status of this folder
      *
      * @param boolean The desired subscription status: true = active, false = not active
      *
      * @return True on success, false on error
      */
     public function activate($active)
     {
         return $active ? kolab_storage::folder_activate($this->name) : kolab_storage::folder_deactivate($this->name);
     }
 
     /**
      * Check subscription status of this folder
      *
      * @return boolean True if subscribed, false if not
      */
     public function is_subscribed()
     {
         return kolab_storage::folder_is_subscribed($this->name);
     }
 
     /**
      * Change subscription status of this folder
      *
      * @param boolean The desired subscription status: true = subscribed, false = not subscribed
      *
      * @return True on success, false on error
      */
     public function subscribe($subscribed)
     {
         return $subscribed ? kolab_storage::folder_subscribe($this->name) : kolab_storage::folder_unsubscribe($this->name);
     }
 
     /**
      * Return folder name as string representation of this object
      *
      * @return string Full IMAP folder name
      */
     public function __toString()
     {
         return $this->name;
     }
 }