Add screenshield and unlock dialog to ctrl-alt-tab

As we now allow the ctrl-alt-tab popup on the lock screen, it should
be possible to navigate back from the top bar, so add the corresponding
elements to the switcher.

https://bugzilla.gnome.org/show_bug.cgi?id=688202

js/ui/unlockDialog.js

@@ -200,6 +200,8 @@ const UnlockDialog = new Lang.Class({
             return false;
         }));
 
+        Main.ctrlAltTabManager.addGroup(this.dialogLayout, _("Unlock Window"), 'dialog-password-symbolic');
+
         this._idleMonitor = new GnomeDesktop.IdleMonitor();
         this._idleWatchId = this._idleMonitor.add_watch(IDLE_TIMEOUT * 1000, Lang.bind(this, this._escape));
     },