Prepare for sfx refactor
Add level up sound path to StatBlock
Deprecate the use of sfx_prefix and make Avatar sounds configurable

Avatar sound configuration is done in `engine/avatar/$GFX_BASE.txt`
Allow modding footstep sound effects

Footsteps are configured in `items/step_sounds.txt`
Merge pull request #1005 from dorkster/sounds

Remove sound effect hard-coding