st: Remove st-group

Now that ClutterActor/StWidget is concrete, we don't need it.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

js/ui/messageTray.js

@@ -1348,9 +1348,9 @@ const MessageTray = new Lang.Class({
             this._onStatusChanged(status);
         }));
 
-        this.actor = new St.Group({ name: 'message-tray',
-                                    reactive: true,
-                                    track_hover: true });
+        this.actor = new St.Widget({ name: 'message-tray',
+                                     reactive: true,
+                                     track_hover: true });
         this.actor.connect('notify::hover', Lang.bind(this, this._onTrayHoverChanged));
 
         this._notificationBin = new St.Bin();