As it turns out, the plasma-settings app is unable to load QWidget-based
plugin UIs, so this needs to be in QML.

It's not yet perfect and should most probably be rewritten to use
StackView instead of a simple Loader, but for now it works.

The original QWidgets code is kept in place, ideally there should be a
detection of the env and show either QML (on the phone) or the QWidgets
(desktop) UI.