A reference to the session bus is now stored in GNotificationBackend.
Remove the extraneous one in the gtk backend and stop using it in
withdraw_notification.
https://bugzilla.gnome.org/show_bug.cgi?id=711051
Move a method from GNotificationBackend into the fdo backend (since it
was only used from here). Remove the accessors for the already-public
(in private header) ->dbus_connect and ->application on
GNotificationBackend.
https://bugzilla.gnome.org/show_bug.cgi?id=688492
