allow the server to export local displays using NX shadow mode

[Antoine Martin]

We can shadow local displays like we do with NX/VNC displays.
The difficulty is in figuring out if that's really a wise thing to do... At least make it global option, preferably let the local user of that display decide (assuming he is running the client...)

We may also want to allow this local display to be exported to via a remote server, which complicates things quite a bit... (reverse SSH tunnel to that server - then another tunnel to the client...)