EMI: Refactor conversion of sound volume values

- LUA code uses a range of 0 .. 100
- the LUA group volumes are in the range 25 .. 100, otherwise the sound
  group is muted
- residualvm engine uses a range of 0 .. Audio::Mixer::kMaxChannelVolume
- do most of the conversion in lua_v2_sound.cpp
- most parts of the engine will solely use the engine internal range
- only when reading/writing the registry residualvm continues to use
  a special conversion for compatibility with the volume settings
  of the residualvm engine settings
- proper handling of volumes close to 0.0f in SoundTrack::updatePosition()
EMI: Set movie volume to SFX volume setting

- movies are played as kPlainSoundType
- set the kPlainSoundType volume to the same value as
  the kSFXSoundType volume