Right now gdm-x-session registers with GDM as soon as the
X server is started, and gdm-wayland-session registers as
soon as the session is started.

Ideally registration wouldn't happen until the session
says things started successfully.

This commit inches us toward that ideal but adding a little
timeout before proceeding with registration.

A future commit will add a new xsession file key to allow
us to know whether or not the session manager of the session
supports doing registration.