Rhythm Quest Update 3 - Music Tracks

No GIF this time since all I worked on was audio stuff.

I now have a handy way of fading AudioSources to a given volume, with an optional callback that executes when it finishes.  Can be used on any AudioSource, you just call AudioSourceFader.Fade(audioSource, targetVolume, speed); and it takes care of the rest.  I'm using this for general music fading in my AudioManager as well now...BUT Rhythm Quest plays three music tracks at once, so that is in Rhythm Quest-specific code -- right now in the "dump it all in" GameController class.  We'll see if it ends up finding another home at some point.

Anyways, now I have a method that I can call on GameController which fades to one of the three bottom/middle/top music tracks that I have set up.  Just using placeholder music songs right now, didn't bother to make anything legitimate yet.

Not sure what's next...either getting the actual tracks set up and getting the player to be able to jump between them, or starting to add some actual notes/obstacles.  Lots of work to be done!  Things are going a lot more slowly than if I was just YOLOing everything, but the code is a bit cleaner as a result...not sure if the tradeoff is worth it to be honest.