layout: rework background handling

This commit updates the code to use mutter's new background
api, and changes the shell's startup animation to be closer
to the mockups.

Based on initial work by Giovanni Campagna

https://bugzilla.gnome.org/show_bug.cgi?id=682429

js/ui/main.js

@@ -10,7 +10,6 @@ const Meta = imports.gi.Meta;
 const Shell = imports.gi.Shell;
 const St = imports.gi.St;
 
-const BackgroundMenu = imports.ui.backgroundMenu;
 const Components = imports.ui.components;
 const CtrlAltTab = imports.ui.ctrlAltTab;
 const EndSessionDialog = imports.ui.endSessionDialog;
@@ -152,10 +151,7 @@ function startSession() {
     windowAttentionHandler = new WindowAttentionHandler.WindowAttentionHandler();
     componentManager = new Components.ComponentManager();
 
-    BackgroundMenu.addBackgroundMenu(global.background_actor);
-
     layoutManager.init();
-    layoutManager.prepareStartupAnimation();
     overview.init();
 
     global.screen.override_workspace_layout(Meta.ScreenCorner.TOPLEFT,