Got wayland subsurface-as-plugin-UI now working for Qt6 hosts now too!

Though because it needs to access private APIs the code looks a bit "special" 😆
But I think it should be possible to expand the supported Qt versions by adding a couple of these...
What do you think, too nasty?

https://github.com/falkTX/wayland-audio-plugin-test/blob/ce3120ba52a1696f5f323948922e281f113cc92b/qt6-host.cpp#L40