MessageTray: add right click menu for summary items

This provides straight forward controls for opening the corresponding
application or removing the summary item.

https://bugzilla.gnome.org/show_bug.cgi?id=617224

js/ui/windowAttentionHandler.js

@@ -101,7 +101,7 @@ Source.prototype = {
         return this._app.create_icon_texture(this.ICON_SIZE);
     },
 
-    _notificationClicked : function(notification) {
+    open : function(notification) {
         Main.activateWindow(this._window);
         this.destroy();
     }