Deallocate space for Window. Window should not be
referred to anymore. Note that Window is not actually
deallocated, but only put on a dead list, which can later be cleaned
clean_up/0. Also note that many things can be destroyed, but
windows are special in that they are not marked as destroyed by this
procedure, but are marked when they get an event indicating that
Window has been destroyed. This is because a window may be
destroyed by other procedures.