Make St aware of the UI group.

Inside the Shell, all the UI (including chrome, the overview, and
the actual windows) is not a child of the stage but of a special
ClutterGroup, which is cloned inside the magnifier.
Add function for setting this special actor so that actors added by
St are visible in the magnifier. Nothing yet uses this, but the
tooltip will soon.

https://bugzilla.gnome.org/show_bug.cgi?id=635100

js/ui/main.js

@@ -136,6 +136,7 @@ function start() {
 
     // Set up stage hierarchy to group all UI actors under one container.
     uiGroup = new Clutter.Group();
+    St.set_ui_root(global.stage, uiGroup);
     global.window_group.reparent(uiGroup);
     global.overlay_group.reparent(uiGroup);
     global.stage.add_actor(uiGroup);