polyphony_time

[kinwie]

This one is to achieve similar function as issue #3 , to make choking (muting) more realistic in SFZ.
Instead of making a new set of choke_group opcodes, we can just add a new polyphony_time opcode, which functioning like off_time, but for each key and tied to group (not off_by), polyphony and note_polyphony

So,

• off_time is used for specifying fade time when off_by other region
• polyphony_time is used for specifying fade time by self-muting when playing a note repeatedly