Allow custom music to be set for battles

[Qiangong2]

Is your feature request related to a problem? Please describe.
Currently (unless I'm missing something), you are unable to set custom battle music. Ideally you'd be able to set custom battle music so that boss fights for example have a different feel.

Describe the solution you'd like
The ability to set custom battle music. Maybe as an extra variable for start_battle?

[JaskRendix]

You can create a new environment in mods/tuxemon/db/environment/ (eg. boss.json, slug = boss, etc.)

eg. https://github.com/Tuxemon/Tuxemon/blob/development/mods/tuxemon/db/environment/grass.json

so you can alter different pieces of the battle scenario (music included: "battle_music": "music_battle_loop"), then you just need to set the variable with:

<property name="act1" value="set_variable environment:boss"/>

but an extra variable in start_battle could be useful