Get rid of deprecated QSignalMapper() in KGoldRunner

Summary:
QSignalMapper() is deprecated

https://build.kde.org/job/Applications/job/kgoldrunner/job/kf5-qt5%20FreeBSDQt5.13/6/warnings12Result/category.2119183113/

this patch replaces it with lambdas.

Test Plan:
1. Compile, install and run KGoldRunner.
2. Start a game and select all menu items in the "Game", "Edit", and "settings" menus.

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: aacid, kde-games-devel

Differential Revision: https://phabricator.kde.org/D23221