main: Call GDM's RegisterSession()
The idea is that we can tell GDM from the Shell (or any other window manager) side when we've finished starting up, so that it can
- Know whether to fallback to Xorg - because the Shell-wayland died without ever registering
- Kill the login screen at the right time, after the session is registered
Here I add a call to this method on the
startup-complete signal of the layout manager. If there's a better place, let me know and it can easily be moved.