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/notificationDaemon.js

@@ -473,7 +473,7 @@ Source.prototype = {
         this._isTrayIcon = true;
     },
 
-    _notificationClicked: function(notification) {
+    open: function(notification) {
         this.openApp();
     },