Add KSignalHandler

Provide a new KSignalHandler class that allows us to catch ANSI signals from our apps and react to them safely.

The code is already in use in plasma-workspace. Having it here will allow other components to use it.

This will allow us to properly handle signals here, as qApp->quit() doesn't guarantee us signal safety: https://invent.kde.org/plasma/kwin/-/blob/master/src/helpers/wayland_wrapper/kwin_wrapper.cpp#L168