Add an explicit method to close kwin
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 and wayland preparation. This leaves kwin to be killed by the display manager as the X connection closes, but this can lead to deadlocks. BUG:428817 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.