EMI: Refactor ranges for sound volume

- LUA code uses a range of 0 .. 100
- the group volumes are in the range 25 .. 100, otherwise that sound
  group is muted
- residualvm engine uses a range of 0 .. Audio::Mixer::kMaxChannelVolume
- do all conversion in lua_v2_sound.cpp
- treat the LUA volume values as opaque when saving/restoring via registry
  (since it is only read/written via LUA code)
- all other parts of the engine will solely use the engine internal
  range
EMI: Set movie volume to SFX volume setting

- movies are played as kPlainSoundType
- set the kPlainSoundType volume according to the
  kSFXSoundType volume