Adjust QtQuickSettings KCM to use KConfigXT in full
This removes quite a lot of uneeded code and fixes the automatic default
handling from KConfigXT point of view (this was the wrong integration
scheme with combo boxes).
It also gives me a nice testing ground for a widget based KCM usable
with kcmshell. :-)
Reviewers: Plasma, mart, davidedmundson
Differential Revision: https://phabricator.kde.org/D25068