Soundtrack menu: set soundtracks without a challenge start sound to not bother playing one

Soundtrack menu: fixed challenge start and hush kill tracks not playing

minor bug fix with demo mode

soundtrack menu:
fixed boss music not playing after bombing angel statue

fixed bug with soundtrack menu where sometimes the music for stage 2 of a boss (e.g. hush) would fail to play properly

soundtrack menu:
-added random options to the Custom soundtrack setting
-fixed mega satan music not being customizable in the Custom soundtrack setting
-fixed music sometimes cutting out while changing custom tracks
-fixed music not restarting in treasure rooms if the default room intro track was played as a result of the current soundtrack not having an intro set
-fixed wrong track sometimes being played when changing to custom mode
-fixed default track sometimes being incorrectly played when starting a new run after opening the game

fixed typo in music.xml preventing antibirth ultragreed music from playing

soundtrack menu:
fixed floor music not starting after treasure room intro