When window initially maps, use the more recent of NET_WM_USER_TIME and
startup notification timestamps to compare against last known user action to
decide whether to focus the window or not. Once we show the window, clear
the initial_timestamp_set flag, so the startup notification timestamp is not
taken into account again.
Based on patch for metacity by Alexander Larsson
http://bugzilla.gnome.org/show_bug.cgi?id=573922
ae32ac86b4