Adjust QtQuickSettings KCM to use KConfigXT in full

Summary:
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

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25068