Introduce `stopped' event in file notification

Introduce `stopped' event in file notification

* lisp/filenotify.el (file-notify--rm-descriptor): New defun.
(file-notify-rm-watch): Use it.
(file-notify-callback): Implement `stopped' event.
(file-notify-add-watch): Mention `stopped' in the docstring.
Check, that upper directory exists.

* test/automated/file-notify-tests.el (file-notify-test01-add-watch):
Add two test cases.
(file-notify-test02-events): Handle also `stopped' event.
(file-notify-test04-file-validity): Add another test case.