Allow changing the length of a note in the piano roll from the start.

Nobody has claimed this yet.

### Enhancement Summary

The only way to change the length of a note in the piano roll is by dragging the end of a note. It would be cool if one could also change the length of a note by dragging the start of the note.

This would useful for creating arpeggiated chords and resizing notes that span several whole notes.

### Implementation Details / Mockup

As of right now, there is a bright green bar at the end of each note. Hovering over it turns the mouse icon into a two headed arrow and allows the user to change the note length. Placing a similar bright bar at the beginning of the note that has the same function would work here as well.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start with the piano roll implementation and trace the existing interaction for dragging a note's end, as described in the issue. Add equivalent behavior at the note's start while preserving the existing end-resize behavior. Done means a visible start handle lets users resize notes from the beginning for arpeggiated chords and notes spanning several whole notes.

Written by the indexing model from the issue text.