Per-note Aftertouch Editor

Nobody has claimed this yet.

### Enhancement Summary

An aftertouch editor, similar to the note pitch editor, would be a useful addition.

#### Justification

In lieu of such, I find myself repeatedly automating the volume knob. This has a few drawbacks: the changes are global to the instrument, not per note (and having different per-note aftertouches presently requires cloning the track as a result). In addition, the timebase for automation clips is relative to the start, requiring constant cross-referencing with the timebase of the piano roll clips, made all more complicated when the two aren't aligned. Finally, because they're not note-attached, they cannot easily be copied with the notes in the piano roll editor, unlike the pitch bend information.

Hopefully no significant major changes need to be made; like the pitch bend editor, this can essentially be just another view of the automation editor. This is arguably a clunky, but workable solution; in addition, if there are ever workflow improvements for the pitch bend editor, the aftertouch editor can automatically benefit.

An open question is how to represent this information in the piano roll. At present, note-attached pitchbend renders a line indicating pitch movement. Since velocity is represented by translucency, it seems the most natural metaphor is changing the opacity over the note--but this might not be particularly easy to quickly visually parse.

No files, tests, or code entry points are named. Start by tracing the existing note pitch editor and automation editor, then determine how per-note aftertouch should be represented in the piano roll; done requires an agreed design and implementation for editing, display, and copying the data with notes.

Written by the indexing model from the issue text.