Request: QMediaPlayer + QAudioProbe support for AV Foundation/macOS

Hello Qt team,

This request is part of the Multimedia component.

Setting the source of QAudioProbe to a QMediaPlayer will always return false on macOS.  Looking at the most recent multimedia backends page here: https://wiki.qt.io/Qt_5.13_Multimedia_Backends it shows that Audio Probe for Media Player functionality is not supported on AV Foundation platforms.

I don't know what it would take to support that functionality, but that would be greatly appreciated if it could made to work alongside the Windows and Unix counterparts.  My use case is creating a simple media player and plotting the waveform that is currently in the buffer, as well as doing some other signal processing for display purposes.

Doing some quick searches through the Apple API, there is an AVAudioBuffer NSObject here, although I don't know what work would be involved in tying in the Qt support for it: https://developer.apple.com/documentation/avfoundation/avaudiobuffer

Thanks for your work!