Overview: Adjust to window size changes

Windows may change their size while the overview is open, e.g. when
switching panels in the control center. Make sure that the preview's
position and overlay are updated in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=640560

js/ui/workspace.js

@@ -1325,6 +1325,10 @@ Workspace.prototype = {
                       Lang.bind(this, function() {
                           this._windowIsZooming = false;
                       }));
+        win.connect('size-changed',
+                    Lang.bind(this, function() {
+                        this.positionWindows(0);
+                    }));
 
         this.actor.add_actor(clone.actor);