messageTray: don't steal focus when popping under the pointer

We must look for the actor under the pointer in the whole message tray and not
just in the notification. This will avoid us to capture focus when a
notification comes up with the pointer on the whole tray area.

https://bugzilla.gnome.org/show_bug.cgi?id=661358

js/ui/messageTray.js

@@ -1860,7 +1860,7 @@ MessageTray.prototype = {
                 // automatically. Instead, the user is able to expand the notification by mousing away from it and then
                 // mousing back in. Because this is an expected action, we set the boolean flag that indicates that a longer
                 // timeout should be used before popping down the notification.
-                if (this._notificationBin.contains(actorAtShowNotificationPosition)) {
+                if (this.actor.contains(actorAtShowNotificationPosition)) {
                     this._useLongerTrayLeftTimeout = true;
                     return;
                 }