Use GDM instead of libaccountsservice for fast user switching

libaccountsservice uses gdmflexiserver internally, which has been
removed from gdm. Instead, use libgdm directly.

https://bugzilla.gnome.org/show_bug.cgi?id=680231

js/ui/unlockDialog.js

@@ -208,7 +208,7 @@ const UnlockDialog = new Lang.Class({
     },
 
     _otherUserClicked: function(button, event) {
-        this._userManager.goto_login_session();
+        Gdm.goto_login_session_sync(null);
 
         this._userVerifier.cancel();
         this.emit('failed');