Audiotaper

This implements a audio taper potentiometer like found in hardware mixer. 
IMHO the new interface and implementation is much more self explaining:

double minDB, 
double maxDB,
double neutralParameter
 
I have made all log pots to this new one, but set the range to the values it was before.
So this is just a refactoring task for now and there should be no notable change from the user point of view. 

In the future this change allows to implement "long" sliders to -70 dB or add a boost region above 1 to a volume slider.   



if m_midicorrection == 127 or m_neutralparameter == 0 this will divide by zero.   we should protect against that in the constructor.