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/runDialog.js

@@ -213,7 +213,7 @@ RunDialog.prototype = {
         // hidden then show it in show()
         this._group = new Clutter.Group({ visible: false,
                                           x: 0, y: 0 });
-        global.stage.add_actor(this._group);
+        Main.uiGroup.add_actor(this._group);
 
         let lightbox = new Lightbox.Lightbox(this._group, true);