Xvnc pre-launch: start other commands but not WM

[Antoine Martin]

At the moment we just pre-launch the Xvnc server process, but not the
"desktop_window_manager_command" or "xvnc_session_commands".

We could pre-launch them anyway (just not the window manager in full_desktop mode)

More difficult than it should be.
Currently, wait_for_session_readyness (which fires the session programs) is only called when starting a session from the controller.
This should be integrated into start_session so that even pre-launch sessions use it.