Avoid a crash on Kwin decoration KCM teardown

Summary:
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.

CCBUG: 373628

Test Plan:
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

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4533