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