on-exit in top-level-window<%>
Called by the default application quit handler (as determined by the
application-quit-handler parameter) when the
operating system requests that the application shut down (e.g., when
the Quit menu item is selected in the main application
menu under Mac OS X). In that case, this method is called for the
most recently active top-level window in the initial eventspace, but
only if the window's
can-exit? method first returns true.