MessageTray: destroy notifications manually when the source is destroyed

Using a signal handlers causes us to depend on connection order, but
we need the message tray code to run last, so it can notice that
notifications are destroyed when hiding the boxpointer and skip
the broken animation.

https://bugzilla.gnome.org/show_bug.cgi?id=686855

js/ui/messageTray.js

@@ -404,11 +404,6 @@ const Notification = new Lang.Class({
         this._soundFile = null;
         this._soundPlayed = false;
 
-        source.connect('destroy', Lang.bind(this,
-            function (source, reason) {
-                this.destroy(reason);
-            }));
-
         this.actor = new St.Button({ accessible_role: Atk.Role.NOTIFICATION });
         this.actor.add_style_class_name('notification-unexpanded');
         this.actor._delegate = this;
@@ -1337,6 +1332,13 @@ const Source = new Lang.Class({
 
     destroy: function(reason) {
         this.policy.destroy();
+
+        let notifications = this.notifications;
+        this.notifications = [];
+
+        for (let i = 0; i < notifications.length; i++)
+            notifications[i].destroy(reason);
+
         this.emit('destroy', reason);
     },