Added battle music Tailchasing and menu theme Headwinds. See notes

Battle music should start when a player aggros a creature. Then it should continue playing until the player stays at least X meters away from any creature. However, the volume of the music should be a linear function of the player's radial distance to the nearest creature: 0% volume at or greater than X meters, 100% at some close distance. All throughout the other background music (Evolving Horizon) should be playing, but its volume should be 100% - the volume of the battle music at any given time.
Merge branch 'master' of https://github.com/allenpark/aquaticevolver