A number of applications want to track the state of the screensaver.
Make this information available as a boolean property. We only listen
for state changes when ::register-session is set to TRUE.

This is implemented for unsandboxed D-Bus access by talking
directly to org.gnome.ScreenSaver or org.freedesktop.ScreenSaver,
and for sandboxed D-Bus by using a (new) portal API.
A Quartz implementation is missing.