wallpapers/image: don't add/remove a file if its parent folder is in KDirWatch

Otherwise KDirWatchPrivate::removeEntry will also remove the parent folder
when the file is removed from KDirWatch.

See https://invent.kde.org/frameworks/kcoreaddons/-/blob/396c75c24ad223bce50302569d42dc4b55bb4580/src/lib/io/kdirwatch.cpp?page=2#L1103

Amends bdbe9b33


(cherry picked from commit 605de29b)