Split up subcategories for KNS categories

This adds functionality to programmatically add subcategories (with a
suitable warning not to do so after the initialisation stage), and
uses that to perform a splitting of the subcategories supplied the
KNewStuff backend.

TEST PLAN
Previously: Many categories, with annoying and bad readability
With patch: A nice dig-down tree of categories in the KDE Store
applications category

Differential Revision: https://phabricator.kde.org/D19925