This also matches the client_window_info.show_hide_level field used in Deskbar
and other applications.
While doing this, keep fShowLevel fully in sync between BWindow and app_server,
use one message type for both hiding and showing, and make the decision to show
and hide the window in the app_server.
Lastly make minimize behave as described in the Be Book: hidden windows cannot
be minimized, and minimized windows which get hidden become unminimized.