ScreenShield: use LayoutManager for creating the actor

This ensures that the screen shield is created at the right
stacking level, so the message tray is visible in the lock screen
(showing PAM messages, critical notifications and the on screen
keyboard)

https://bugzilla.gnome.org/show_bug.cgi?id=619955

js/ui/layout.js

@@ -40,6 +40,17 @@ const LayoutManager = new Lang.Class({
         this.panelBox.connect('allocation-changed',
                               Lang.bind(this, this._updatePanelBarriers));
 
+        this.screenShieldGroup = new St.Widget({ name: 'screenShieldGroup',
+                                                 visible: false
+                                               });
+        // FIXME: This is going to slow everything down by a big factor
+        // Need a better solution (like a layout manager?)
+        let constraint = new Clutter.BindConstraint({ source: global.stage,
+                                                      coordinate: Clutter.BindCoordinate.ALL
+                                                    });
+        this.screenShieldGroup.add_constraint(constraint);
+        this.addChrome(this.screenShieldGroup);
+
         this.trayBox = new St.BoxLayout({ name: 'trayBox' }); 
         this.addChrome(this.trayBox);
         this.trayBox.connect('allocation-changed',