vertical keyboard navigation 

There are two components to this change:

1) I added next-track and prev-track commands that move the cursor to the next or previous non-empty track at the current tick position.  If the target track is non-empty but contains nothing at the current tick position, cursor is moved backward to the previous note or rest on that track.

2) I changed the behavior of the existing down-chord and up-chord commands.  Previously, these moved through the current segment from highest to lowest note regardless of which track they were found on, and they skipped tracks that had rests or nothing at that tick position.  The new behavior is more consistent with what I believe most users might expect: down-chord and up-chord move note by note through the current chord on the current track, and then onto the next or previous track as with the new next-track and prev-track commands.