A little milestone achieved, Carla is working with Qt6 now! 🎉

I wish to be doing other things than updating to new frameworks to stay relevant, but at least Qt5 -> Qt6 compatibility is quite okay.
The annoyance is more on PyQt side that loves to break backwards compat. And Python somewhat too.

I regret having chosen Python for the frontend but a bit too late now since there is quite a lot of code...