shutdown: Introduce support for KWin closing wayland windows

After ksmserver is done, KWin will now close (or not) open wayland windows. Since this operation can also cancel the logout, ksmserver gains a new hook to reset its state. This also means that it cannot quit itself anymore but needs to be managed by plasma-shutdown. Using this opportunity the path for shutdown can be aligned between X11 and Wayland. In the systemd-managed case we stop graphical-session.target, otherwise plasma-shutdown quits KWin and ksmserver.