This PR adds 3 types of audio streams used for interactive music support.
* AudioStreamInteractive: Allows setting several sub-streams and transition between them with many options.
* AudioStreamPlaylist: Allows sequential or shuffled playback of a list of streams.
* AudioStreamSynchronized: Allows synchronous playback of several streams, the volume of each can be controlled.
Theese three stream types can be combined to create complex, layered interactive music and transitions between them, similar to software such as WWise.
Adam Scott
Juan Linietsky