Upgrade QMidi to be compatible with Qt 6.2

[Paul Masri-Stone]

I have done this and will submit a PR.

Following the Porting to Qt 6 guide, I first compiled with Qt 5.15 disabling deprecated functionality. This showed that qStableSort() has been deprecated in favour of std::stable_sort.

The only other change needed is the CMakeLists.txt which is specific to Qt 5.

[waddlesplash]

Thanks!