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/chrome.js

@@ -35,7 +35,7 @@ Chrome.prototype = {
     _init: function() {
         // The group itself has zero size so it doesn't interfere with DND
         this.actor = new Shell.GenericContainer({ width: 0, height: 0 });
-        global.stage.add_actor(this.actor);
+        Main.uiGroup.add_actor(this.actor);
         this.actor.connect('allocate', Lang.bind(this, this._allocated));
 
         this._inFullscreen = false;