Allow access the upcoming queue items, and inserting CoreAudioStreamSource items after a particular item in the queue

[jacksonh]

Two things here:

1. I needed some access to the queue to be able to visualize it without trying to keep two separate lists in sync, I think it makes most sense if AudioPlayer is the source of truth for most apps.
2. Ability to insert after so we can re-order CoreAudioStreamSource items in the queue

[jacksonh]

Hey, I don't need this change anymore, I decided to move my queue handling outside of AudioStreaming which gave a bit more control. I'll leave the PR open though, let me know if you think its useful and if there's anything you'd like changed.