workspace: Focus clone when highlighting

We don't use different hover- and focus indications for window previews,
so using keyboard navigation after hovering a clone often has surprising
results when the previous focus window wasn't the selected one. Address
this by simply moving the keyboard focus with the highlight.

https://bugzilla.gnome.org/show_bug.cgi?id=786546

js/ui/workspace.js

@@ -662,6 +662,7 @@ var WindowOverlay = new Lang.Class({
         if (this._hidden)
             return;
 
+        this._windowClone.actor.grab_key_focus();
         this._animateVisible();
         this.emit('show-close-button');
     },