Add an explicit quit mechanism controlled by plasma
On X Kwin's lifespan used to be managed by XSMP. This accidentally broke in 5.20 as we made kwin start before ksmserver for speed purposes.
This leaves kwin to be killed by the display manager as the X connection closes, but this can lead to deadlocks.
On wayland kwin's lifespan is mapped to the lifespan of the ksmserver binary. This is problematic as it makes our entire xwayland robustness redundant if we ultimately rely on an application that /has/ to run in xwayland.
BUG: I'm sure there is one, I read it commented on it, but I can't find it.