xpra full desktop using Xnest

[Antoine Martin]

This is far from ideal: we end up with 2 extra X11 servers (the Xvfb and Xnest running inside it) and performance is not going to be great.
But this is fairly easy to add, so why not?

Server must tell the clients that it supports this mode (==it has Xnest installed and this is enabled in config).
Then on app start we run Xnest and use the old port probing code to launch the real app against the Xnest display.
(not going to do pre-launch mode for this - not worth it)

[Antoine Martin]

done in r1763

including pre-launch as it was quite easy to share the code, see: xnestify()

We should still do:

• kill Xnest when application terminates (similar to delayed-start for vnc)
• shadowing via NX on the Xnest display (doesn't work on the Xvfb)