screenShield: add initial functionality

We are replacing the gnome-screensaver module with with a screen shield
that is part of gnome-shell.

This patch fades out the screen on idle and shows a shield with a background
image when there is activity again. The shield can be removed with a key or
button press.

js/ui/main.js

@@ -29,6 +29,7 @@ const LookingGlass = imports.ui.lookingGlass;
 const NetworkAgent = imports.ui.networkAgent;
 const NotificationDaemon = imports.ui.notificationDaemon;
 const WindowAttentionHandler = imports.ui.windowAttentionHandler;
+const ScreenShield = imports.ui.screenShield;
 const Scripting = imports.ui.scripting;
 const ShellDBus = imports.ui.shellDBus;
 const TelepathyClient = imports.ui.telepathyClient;
@@ -52,6 +53,7 @@ let runDialog = null;
 let lookingGlass = null;
 let wm = null;
 let messageTray = null;
+let screenShield = null;
 let notificationDaemon = null;
 let windowAttentionHandler = null;
 let telepathyClient = null;
@@ -211,6 +213,7 @@ function start() {
     panel = new Panel.Panel();
     wm = new WindowManager.WindowManager();
     messageTray = new MessageTray.MessageTray();
+    screenShield = new ScreenShield.ScreenShield();
     keyboard = new Keyboard.Keyboard();
     notificationDaemon = new NotificationDaemon.NotificationDaemon();
     windowAttentionHandler = new WindowAttentionHandler.WindowAttentionHandler();
@@ -670,6 +673,10 @@ function _findModal(actor) {
     return -1;
 }
 
+function isInModalStack(actor) {
+    return _findModal(actor) != -1;
+}
+
 /**
  * pushModal:
  * @actor: #ClutterActor which will be given keyboard focus