More on channels management

Summary:

1. Added channels list widget (with Create button)

with small refactoring in handling list widgets and their titles
no backend changes yet to actually get the list of channels

2. Create Chat room details widget

.. for displaying chat information and e.g. update it's name
Not much backend support yet

Reviewers: Kolab Chat Developers

Subscribers: seigo

Differential Revision: https://git.kolab.org/D443