removed AudioQueue and added direct IO callback

[konradglas]

MacOS Only:

• Replaces the use of AudioQueue with a direct callback from CoreAudio
• Goal of this change is to have more control over and reduce latency
• And direct control over subsequent threading

Notes/Assumptions:

• Core Audio callback expects Float32 format to be put inside its OutputBuffer
• We will always request 2 channels
• Device Stopping happens inside the audio callback. This is a bit unusual, but I found the same pattern in a widely used audio framework. It should at least guarantee that after the isStopped flag is set, the callback will not be called anymore and the used Data can be cleared.