Check if new files occurred while the watcher was disabled

If new files were added to the watched folders while the watcher was
disabled then the watcher didn't watch those new files for changes. On
the very first start with an empty GNUPGHOME this resulted in trustdb.gpg
not being watched (which caused problems when an imported secret key was
marked as ultimately trusted). Similarly, when importing secret keys or
generating new keys the added secret key files were not watched.

This is fixed by explicitly checking for new files whenever the watcher
is re-enabled.

• GnuPG-bug-id: T8015