messageTray: Inherit Notification, Source and NotificationPolicy from GObject

Register notifications, sources and policies as GObject gtypes so that they can
be passed in signals and use native properties and signals.

Reimplement all the extending classes.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559

js/ui/overview.js

@@ -72,7 +72,7 @@ var ShellInfo = class {
         if (undoCallback)
             notification.addAction(_("Undo"), this._onUndoClicked.bind(this));
 
-        this._source.notify(notification);
+        this._source.showNotification(notification);
     }
 };