Gtk UI: improvements in search widget.
Gtk UI: tiny fix
Qt UI: use sidebar by default.
Qt UI: some improvements in settings dialog:
hide options which do not work in Mac OS X, MS Windows or Haiku.
Qt UI: disable system tray functionality in Mac OS X.
Qt UI: improvements for Mac OS X:
change program icon in dock when there are new unread personal messages.
Qt UI: improvements for Mac OS X:
* add special program menu to the dock icon
* hide main window by close button instead of minimizing it