Limiter:re-scaling proposal of Threshold values, to get 0 dB as 'max.value' instead of 20 dB

Issue #609
Limiter:re-scaling proposal of Threshold values, to get 0 dB as 'max.value' instead of 20 dB
StereoSoundFiler: outlevel range is 0-4 , not 0-1 (as usual)
Edit

Issue #624 - not only StereoSoundFiler "soundLvevel" slider needs to be called gain but also:
AddSynth.module
Flute.module
FourByOneMixer.module
MultiBandCompressor.module
OctoSoundFilePlayer.module
PartialAnalyser.module
PluckedString.module
Soundfiler.module
StereoSoundfiler.module
StereoSoundfileTrigger.module
SubSynth.module
TestSource.module