implement exponential midi Note-On velocity response

[rghvdberg]

From the Midi 1.0 specification

VELOCITY
Interpretation of the Velocity byte is left up to the receiving instrument. Generally, the larger the > numeric value of the message, the stronger the velocity-controlled effect. If velocity is applied to volume (output level) for instance, then higher Velocity values will generate louder notes. A value of 64 (40H) would correspond to a mezzo-forte note and should also be used by device without velocity sensitivity. Preferably, application of velocity to volume should be an exponential function.

relevant code
https://github.com/rghvdberg/ninjas2/blob/develop/plugins/Ninjas2/Ninjas2Plugin.cpp#L908