Instead of having various flags for realizing the arpeggion functionality
use a more generic approach here using the recently introduced "origin"
property.
The volume of an InstrumentTrack is applied separately when post-processing
the audio buffer and is not related to MIDI processing. It therefore should
not be included into MIDI velocity calculation.
Closes#301.
Next big coding style update - this time all PlayHandle classes are
affected. Functions like done() and released() were renamed to
isFinished() and isReleased().