Usage: SET STATUS_NOTIFY <Status line text>
  The contents of STATUS_NOTIFY are replaced in the STATUS_FORMAT
  variable for any occurence of %F when there has been activity in
  a hidden window. STATUS_NOTIFY may contain occurences of %F which
  will be replaced by the list of windows in which there has been
  activity. If there has been no activity in hidden windows, %F
  is replaced by nothing in the STATUS_FORMAT.