Unit tail handling

[olofson]

We need mechanisms for handling reverb tails and the like! Detached voices terminate when all their subvoices have finished, ignoring tails of the local units, which is obviously not very nice at all.

• It would be easy enough to add a VM instruction that looks at the output - but that doesn't work when the last unit mixes directly into an external output buffer.
• Use an envelope tracker unit? Set it up to measure the peak value over some suitable period of time, and poll the result...
• The most efficient solution is probably to have units report their tail status via some efficient out-of-band interface.