messageTray: Notify on notification updates

Since notification support was added to the lock screen, notifications
are no longer necessarily represented by the actual notification actor
anymore. However when an existing notification is updated, external
representations currently become outdated.
Emit an appropriate signal which allows them to update.

https://bugzilla.gnome.org/show_bug.cgi?id=744817

js/ui/messageTray.js

@@ -677,6 +677,7 @@ const Notification = new Lang.Class({
         }
 
         this.updated();
+        this.emit('updated', params.clear);
     },
 
     setIconVisible: function(visible) {