Use QQuickWidget instead of QQuickView for the main view in the kcm
Summary:
This makes it usable on wayland. The Qt Wayland support for subsurfaces
is broken in many ways, producing practically unusuable graphics and crashes.
https://bugreports.qt.io/browse/QTBUG-54888 for instance.
Test Plan: Used it on wayland works fine now.
Reviewers: Plasma, sebas, dvratil, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11045