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