Ongoing work on repeated music

[Dan Eble]

Lately, I've been trying to contribute self-contained, well-tested enhancements 
to repeated music in bottom-up fashion.  In the code review for the `\section` 
command, Harm expressed interest in what's still in the queue.  I don't want to 
clutter the review with tangential discussion, so I'm providing a summary here.

The goal is a `\repeat segno …` command that provides useful automatic 
engraving and performance for common cases involving D.S., D.C., al Fine, al 
Coda, etc.  This is built on other features that could be used on their own:

Generalize `\inStaffSegno` into `\segnoMark N` (much like `\mark N`) with 
context properties controlling how it is engraved.  I don't anticipate much 
direct use of `\segnoMark` by users, but it is a building block for `\repeat 
segno …`.

Add a `\fine` command which engraves a `|.` bar line, engraves a "Fine" 
performance instruction, and does the obvious when repeats are unfolded.  This 
could be used with volta repeats as well as segno repeats.

To support D.S. al Coda, add a command to indicate that a repeat alternative 
should be treated as the Coda.  At a minimum, this would be something like 
`\codaSection`, but it would be nice to generalize the idea of naming a section 
or repeat alternative if it doesn't overcomplicate the implementation of 
`\repeat segno`.

`\repeat segno` has been "basically working" for a while, but there are aspects 
that I would like to be more automatic or configurable and there remains a lot 
of tidying-up, testing, and debugging.

Regards,
— 
Dan