Change ‘directory’ to ‘folder’ in user-visible strings

Try and enforce consistency in the use of folder-related vocabulary in user-visible strings.

To discuss the vocabulary choice criteria itself, please refer to https://invent.kde.org/websites/hig-kde-org/merge_requests/62

(I’ve sent an email to ufleisch@users.sourceforge.net since I could not find his KDE GitLab username)