Give KNSRC file a user-friendly name

Summary:
With this change, the catagory name in Discover will be "K3B Themes" instead of "K3Btheme".

See also b3928bb58819c4306a23918762be7cd5aa663979

BUG: 400589
FIXED-IN: 19.04.0

Reviewers: #kde_applications, apol

Reviewed By: apol

Differential Revision: https://phabricator.kde.org/D17637