StatusNotifierItem ("D-Bus tray icon") improvements

* Get rid of the serviceOwnerChange() deprecation warning in Qt >= 4.6
* Fix behavior when the StatusNotifierHost (dis)appears. Thanks to notmart for
  fixing this in KDE as well; note that older versions of KDE's SNWatcher won't signal
  when the host goes away, so Quassel won't go into legacy mode!
* Fix hiding of the tray icon in StatusNotifier mode. No more restarting needed.