Support custom MIDI velocity curve

Hello,

For now velocity for midi input can be raw from keyboard or fixed to a value.
It's ok with some keyboards but for keyboards with fixed velocity curve it does not work well.
A better (simple) approach could be to have 2 values, min and max velocity.
A much better approach should be to have a real 1->127 to 1->127 graphical editor to create a mapping curve.
For now and for my needs (with an Akai LPK25 mini keyboard) I just use the fixed value as a minimum velocity value:

void MidiPort::processInEvent( const MidiEvent& event, const MidiTime& time )
...
if( fixedInputVelocity() >= 0 && inEvent.velocity() < fixedInputVelocity() ) // FIXME GFE now input is not fixed but minimum
{
inEvent.setVelocity( fixedInputVelocity() );
}

With this simple hack velocity is useable (with a value of 50). We absolutly need to limit the range of velocity because it seems most instrument use velocity as a volume (but that is another debate...).

The relevant entry point shown is MidiPort::processInEvent; start there and review how fixedInputVelocity() and inEvent.velocity() currently interact. Before changing it, settle whether the scope is min/max limits or a graphical 1-to-127 mapping; done should mean the agreed velocity curve is applied to MIDI input within the valid range.