loginDialog: don't show Not Listed? button before user list

Right now, there's a weird flicker at start up where the
Not Listed? button shows up before the user list, which looks
pretty bad if you're watching for it.

This commit fixes that problem by hiding the Not Listed button
initially and showing it at the appropriate time.

https://bugzilla.gnome.org/show_bug.cgi?id=703132

js/gdm/loginDialog.js

@@ -621,6 +621,7 @@ const LoginDialog = new Lang.Class({
                                                 x_fill: true });
 
         this._notListedButton.connect('clicked', Lang.bind(this, this._hideUserListAndLogIn));
+        this._notListedButton.hide();
 
         this._userSelectionBox.add(this._notListedButton,
                                    { expand: false,
@@ -1153,6 +1154,7 @@ const LoginDialog = new Lang.Class({
     _showUserList: function() {
         this._hidePrompt();
         this._setUserListExpanded(true);
+        this._notListedButton.show();
         this._userList.actor.grab_key_focus();
     },