Add kde-applications to the supported kde-<categories>

This adds kde-applications to categories that can be used to setup
MASTER_SITES and such for kde ports.

With that we are now ready to update KDE4 and look at importing at least
some of the KF5 based aplications from the testing repo.

Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D8893