Add an audio player that can handle background music dynamically

[GerryDE]

• Should be able to play a single audio track as background music
• Should be able to loop the track jumping to a given loop point (either in PCM samples or seconds) once a given time position is reached in the track
• Should be able to dynamically fade into another track. Could be achieved by using a second AudioSource that fades into the new track while the former track is fading out
• Should be able to react on events and start fading into other tracks based on the event (e.g. fade into battle music when enemies appear or into boss music when a boss appears)