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