kinda-sorta dynamic music yay

--Added some new BGM clips to the Assets
--Added a method isTreeBossOnScreen() to levelController.cs for use in
DemoBGMplayer.cs
--Created new script DemoBGMplayer.cs that plays music for the demo
level (music changes after boss appears and after currently playing loop
ends; if i have time on saturday/sunday I'll make it change immediately
when the boss is onscreen)