Fix crash on startup

Summary:
There was various threads using the same kconfig[group] at the same time, so bad things happened.
Add a mutex

BUGS: 398416

Test Plan:
Run
  rm -f ~/.config/palapeli* && palapeli
a few times, see it doesn't crash anymore

Reviewers: #kde_games

Subscribers: chehrlic, kde-games-devel

Differential Revision: https://phabricator.kde.org/D16118