messageTray: Don't always open the source when clicking on the notification

Some consumers may not always want to open the app, so make clients that
want to do this explicitly connect to the 'clicked' signal.

https://bugzilla.gnome.org/show_bug.cgi?id=710137

js/ui/messageTray.js

@@ -1385,9 +1385,7 @@ const Source = new Lang.Class({
         if (this.notifications.indexOf(notification) >= 0)
             return;
 
-        notification.connect('clicked', Lang.bind(this, this.open));
         notification.connect('destroy', Lang.bind(this, this._onNotificationDestroy));
-
         this.notifications.push(notification);
         this.emit('notification-added', notification);