Graphical remote login through GDM

It should be possible to use gnome-remote-desktop to login remotely using the GDM login screen. Logging in should either take over an existing physical session (see #91) or create a new headless session (#90 (closed)), one that either can turn into a physical one, or one that is intended to be only remote.

When logging in, a remote desktop connection from the login screen should be handed over to a remote desktop server running as the logged in user.