Fix crash when using QString as KConfigGroup name, use C Char array from start

Fixes https://bugs.kde.org/show_bug.cgi?id=456157 , seems something goes awry in KConfigGroup's constructor _here_: https://invent.kde.org/frameworks/kconfig/-/blob/master/src/core/kconfiggroup.cpp#L471

Closes https://invent.kde.org/pim/kalendar/-/issues/204



Signed-off-by: Claudio Cambra <claudio.cambra@gmail.com>