Optionally, don't use file system watchers

The usage of file system watchers can be disabled by setting
KLEO_NO_FILE_WATCHER=1. This is mostly useful for debugging problems
that might be related to the file system watchers. Usually, the file
system watchers should be used so that Kleopatra updates the list of
certificates whenever something changed.

• GnuPG-bug-id: T8181