Progress -- but buggy

Workers are still not defined, and there are bugs
which have to be fixed first.
This commit contains some special comments
in Tracks.js and Sequence.js (beginning with
"// *** ") as notes for how the workers might
eventually be used.
Currently debugging just with a single channel
(Song Six, wind 2). This should be working before
before beginning to write any real worker code.
Single track now working correctly. Known bugs.

Currently testing with the existing Song Six score.
Multiple tracks are not working correctly yet.
Good, radical changes. Non-live nearly working.

The code is now much cleaner and more elegant. Song Six now plays back correctly (if the R2M
channel is disabled).
Web workers are not (yet) required.
TODO_1: debug with the R2M channel (which
alone contains silent chord moments).