RFC: EMI sound volume fixes

Cleanup of the volume handling:
* EMI and the mixer using different ranges
* do all conversion in lua_v2_sound.cpp instead of having some volume variables using EMI's range and other the mixer range
* special conversion in registry is retained for compatibility with the sound volume settings when using the residualvm UI
* remove hard-coded volume values

Videos are now played with the SFX volume (as in the retail version):
* it is now possible to mute videos

Maybe add a comment saying that these are helper functions, and not called from lua directly. (although, that should be obvious looking at them.