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.   



The old version was setting the step by a const value offset. I have introduced setStepCount, to have equal normalized steps on a logarithmic potmeters. 
Here we have a value range from 1 ... 200 = 199 steps.