send loop trigger

[catfact]

would be helpful to have a non-periodic poll that simply fires when a loop happens in softcut (with voice ID number perhaps)

the old ugen did this.

requires:

• each audio block, each voice, each subhead reports boolean didLoop
• add new Poll and update appropriately: this means we need proper mechanism for non-periodic polls with updates initiated from audio thread. this could be a worker thread or a non-blocking, non-allocating lo_send analog.