Revert porting to new qmlRegisterInterface variant, needs careful rework

The "Plasma" namespace of the classes registered makes this not a straight-forward port, instead things got broken. And my testing was screwed, I might have tested and run the days an unpatched installation.

There are some ideas how the porting should be done instead and what needs fixing. But that should be done with proper care and as there is no need to have that code ported now away from the deprecated qmlRegisterInterface variant, even more for upcoming 5.72, just reverting to the old working code takes out any pressure.