Migrate session action shortcuts out of ksmserver

As part of a mission to make ksmserver only about X11 session management, this patch splits the session actions out of ksmserver.

Making ksmserver optional at runtime is a pre-requisite for making xwayland only load on demand.

This patch uses kglobalaccel's ability to load actions directly from a desktop file.

We can go with a kded module approach if that's preferred or we hit any other limitations.