Add contrib/magit-filenotify.el.

Emacs (bzr) now comes with a more general file notification
system (filenotify.el).  Unlike inotify it also works on w32 and with
glib support.  This module is a replacement for
contrib/magit-inotify.el.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>