We're first getting the notification by its notify_id, but activating
the action afterwards could redraw it, leading to a dangling pointer.
Fix this by simply searching the list of active notifications again
after activating the action.
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
