Fix background to animate with thumbnails

When we animating the scale for the thumbnails, the border and
background should wrap around the current size of the thumbails.
The technique that we are using to animate the scale breaks that
since we don't animate the overall size of the thumbnails box -
we just animate our child actors within the allocation.

To fix this, switch from drawing the background by packing in another
container to drawing the background with a separate actor that
is under the other actors and allocated by our custom logic.

https://bugzilla.gnome.org/show_bug.cgi?id=641881

js/ui/workspacesView.js

@@ -621,7 +621,8 @@ WorkspacesDisplay.prototype = {
 
         let controls = new St.Bin({ style_class: 'workspace-controls',
                                     request_mode: Clutter.RequestMode.WIDTH_FOR_HEIGHT,
-                                    y_align: St.Align.START });
+                                    y_align: St.Align.START,
+                                    y_fill: true });
         this._controls = controls;
         this.actor.add_actor(controls);