osdWindow: Make sure the OSD is always our topmost chrome element

In particular this fixes the OSD showing up behind a modal dialog's
lightbox.

https://bugzilla.gnome.org/show_bug.cgi?id=701269

js/ui/osdWindow.js

@@ -138,6 +138,7 @@ const OsdWindow = new Lang.Class({
         if (!this.actor.visible) {
             this.actor.show();
             this.actor.opacity = 0;
+            this.actor.get_parent().set_child_above_sibling(this.actor, null);
 
             Tweener.addTween(this.actor,
                              { opacity: 255,