Consolidate creation of login and unlock dialog in the screenshield

The design calls for the curtain to appear in the gdm greeter too.
Implement this by having the screenshield manage the login dialog
(delegating its creation to SessionMode).

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

js/ui/unlockDialog.js

@@ -7,7 +7,6 @@ const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Gtk = imports.gi.Gtk;
 const Lang = imports.lang;
-const Mainloop = imports.mainloop;
 const Signals = imports.signals;
 const Shell = imports.gi.Shell;
 const St = imports.gi.St;
@@ -152,6 +151,11 @@ const UnlockDialog = new Lang.Class({
 
         this._updateOkButton(false);
         this._reset();
+
+        GLib.idle_add(GLib.PRIORITY_DEFAULT, Lang.bind(this, function() {
+            this.emit('loaded');
+            return false;
+        }));
     },
 
     _updateOkButton: function(sensitive) {