Reorganize stage in terms of a UI Group actor and everything else

In preparation for adding magnification, "uiGroup.patch", organizes the stage
along the following lines:

Stage
  *Magnifier
  UI group
    Window group
    Chrome group
    Overlay group
    Alt tab
    App display
    Chrome
    ...

This allows a magnifier actor to clone and magnify the UI group.  The magnifier
is a sibling of the UI Group in this stage oraganization -- see the next patch,
"Magnifier.patch".

js/ui/panel.js

@@ -605,7 +605,7 @@ Panel.prototype = {
                                    transition: 'linear',
                                    onUpdate: function() { ripple.opacity = 255 * Math.sqrt(ripple._opacity); },
                                    onComplete: function() { ripple.destroy(); } });
-        global.stage.add_actor(ripple);
+        Main.uiGroup.add_actor(ripple);
     },
 
     _onHotCornerEntered : function() {