Even if "low resource usage mode" is turned on, which says it avoids
storing thumbnails to disk to save space, it still stores thumbnails to
disk. It just deletes _all_ thumbnails afterwards (even those not from
gwenview itself).

This was also kind of racy, because thumbnailwriter could still be
running, but the folders would be deleted, so it spammed a ton of error
messages.

Test Plan:
Doesn't store thumbnails when it shouldn't anymore, doesn't
delete thumbnails from other applications nor the thumbnail folders,
thumbnailwriter doesn't spaz out during exit, stores thumbnails when it
should.

Differential Revision: https://phabricator.kde.org/D28375