[Audio Export API] Sync Point Handling

[Danielku15]

This feature handles the audio export which respects a custom sync point setup.

If the goal is to synchronize alphaTab with external systems (e.g. audio or video) you want to rather adjust the audio generated by alphaTab and keep the external system untouched.

e.g. if you have a video with a backing track, and you don't want to change the video, you can rather change alphaTab audio to match the video (via sync points).