New variable last_user_time.

New variable last_user_time.
(handle_one_xevent): Set last_user_time from events that have Time.
Set net_supported_window to 0 when reparented.
(wm_supports): New function.
(do_ewmh_fullscreen): Use wm_supports to check for _NET_WM_STATE.
(x_term_init): Initialize net_supported_atoms, nr_net_supported_atoms
and net_supported_window.