Fixes for #33, #34 and #38

Fixes for issues #33 (MIDI hold pedal), #34 (internal keyboard velocity) and #38 (playing more than 8 notes causes maximum velocity retriggers).

In fixing #38, I've also changed the assigned behavior so that restored notes are assigned in legato mode not only for monophonic modes but also in poly mode. That way a note isn't retriggered when reassigned as the result of releasing another key, the pitch just changes, which I think is what you'd expect to happen.