layout: add chrome actors directly to uiGroup

Rather than having a single chrome layer and putting all of the chrome
into that, put the chrome actors directly into uiGroup, so that they
can be stacked independently of one another relative to other actors.

(This requires making uiGroup a ShellGenericContainer, so we can use
skip_paint to avoid painting non-visibleInFullscreen chrome when we're
in fullscreen.)

https://bugzilla.gnome.org/show_bug.cgi?id=657986

js/ui/main.js

@@ -193,7 +193,13 @@ function start() {
     loadTheme();
 
     // Set up stage hierarchy to group all UI actors under one container.
-    uiGroup = new Clutter.Group();
+    uiGroup = new Shell.GenericContainer({ name: 'uiGroup' });
+    uiGroup.connect('allocate',
+                    function (actor, box, flags) {
+                        let children = uiGroup.get_children();
+                        for (let i = 0; i < children.length; i++)
+                            children[i].allocate_preferred_size(flags);
+                    });
     St.set_ui_root(global.stage, uiGroup);
     global.window_group.reparent(uiGroup);
     global.overlay_group.reparent(uiGroup);