Fix handling of different signals (SIGTERM et al) by using KSignalHandler

When debugging something I realised that SIGTERM was always making KWin
crash like I'd never seen it crash. It turns out we are calling
QApplication::exit() on the signal handler which is highly forbidden as
the handler preempts the process altogether.
Use KSignalHandler instead which takes this issue into account.


(cherry picked from commit d7cba6b6)