kns: Show subcategories for things that have one

Summary: Show subcategories for KNS resources that have more than 1 Category inside. At the moment it just is Wallpapers.

Reviewers: #discover_software_store, leinir

Reviewed By: leinir

Subscribers: leinir, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19826