Qt6 patch

[edpil02]

I made a qt6 version of Carla.
Need help to improve my patch.
carla.zip

[edpil02]

[falkTX]

Thanks for the patch! this will help with the transition.

But I am not happy about updating to Qt6 right now, it requires updating the codebase to C++17 and drops support for many older OSes.
Qt5 still works fine so it will remain the default in Carla for the time being.

But I will make it so Qt6 build is possible.