Add target "Track: Enable/disable parent send"

[helgoboss]

This seems to be useful for song/sound switching use cases:

At the moment, I do this by arming/disarming a track which has its input set to the desired MIDI device. Sometimes, I put a VSTi (or hardware output send) directly on that track. At other times (if I want the same sound to be controlled by multiple keyboards/zones), I put the VSTi on the parent track. In both cases, disarming the track sends note-off messages, which is important to prevent hanging notes.

It could be useful to use sends instead, so that not each track has to reference MIDI input devices. Referencing them only once has the benefit that we can quickly change which input device is used, we also wouldn't need to mess with Device IDs when opening the setup on another machine. Song/sound switching would then be done by unmuting/muting MIDI sends. Unfortunately, muting a MIDI send doesn't send note-off messages.

However, there's an alternative that's even more flexible: Always adding the track as a child of the VSTi track. Creating an always unmuted send from the device input track to that child track. And then enabling/disabling parent send. This seems to send note-off messages!