WindowOverlay: Show close button after an animation

If a user is fast and mouses over a window while the workspace thumbnail
animations are playing, it can be frustrating when the close button won't
appear at the end of the animation.

https://bugzilla.gnome.org/show_bug.cgi?id=645848

js/ui/workspace.js

@@ -394,6 +394,8 @@ WindowOverlay.prototype = {
 
     show: function() {
         this._hidden = false;
+        if (this._windowClone.actor.has_pointer)
+            this.closeButton.show();
         this.title.show();
     },