This commit splits ksmserver's xsession shutdown logic from performing
the actual shutdown and running shutdown scripts and implement proposed
Intended longer term target is to move this to a separate executable.
KSMServer's existing logout dbus method still exists for compatibility
forwarding to the new interface.
There are 2 minor behavioural changes.
The shutdownMode property (which doesn't seem to do anything and is not
exposed in our UI) is not kept.
If you shutdown /whilst/ starting up somehow, previously we delayed
showing the logout prompt, we now delay performing the actual logout.