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)

wait_for_session_readyness which fires "prepare_session" should only be called once, preferably early (during pre-launch).