Add a mimeType() method to avoid inconsistency which one is used (old one is text/x-vard, new officical one one text/directory).

Also makes it more consistent with newer classes, e.g. KABC::ContactGroup or Akonadi::Collection which have such a method since Akonadi has increased usage of MIME types for all kinds of indentification tasks

svn path=/trunk/KDE/kdepimlibs/; revision=925827