Avoid a crash on Kwin decoration KCM teardown
As described in https://bugreports.qt.io/browse/QTBUG-57714 exposing a
QWidget as a context item to QtQuick can crash. Especially as the
engine's context property gets deleted whilst deleting the parent item.
This patch reworks the code so that the models are exposed to QML
directly rather than going through a QWidget.
Pressing back whilst in the decoration KCM used to crash every time, now it doesn't.
I still have the buttons.
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: graesslin, cfeck, plasma-devel
Differential Revision: https://phabricator.kde.org/D4533