KF5 port: remove deprecated KStringHandler and KTabWidget usage

Summary:
KStringHandler::naturalCompare() was replaced with QCollator (according to deprecating comment, other commits did it the same way)
KTabWidget was replaced with QTabWidget - there were no special KTabWidget functions at all.

Test Plan: TriggerConfig works as before

Reviewers: #kde_games, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, #kde_games

Differential Revision: https://phabricator.kde.org/D7417