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 up by 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.