messageTray: Hide notification close button immediately on click

Having the close button move away from under the pointer after
clicking it is confusing and distracts from the main transition,
which is hiding the notification. Just hide it immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=682237

js/ui/messageTray.js

@@ -1588,6 +1588,7 @@ const MessageTray = new Lang.Class({
 
     _onCloseClicked: function() {
         if (this._notificationState == State.SHOWN) {
+            this._closeButton.hide();
             this._notificationClosed = true;
             this._updateState();
             this._notificationClosed = false;