Issue 3799: New satb.ly built-in template and template framework
Add kits as aids for providing built-in templates ("tkits")
Replace the satb.ly template with one built from the tkits, which
corrects some errors and extends the facilities:
- fixes issue 4192
- \lyricsmode should be used; never \lyricsto
- Women and Men one-voice staves added
- Two-voice staves called MenDivided, WomenDivided
- Soprano and Tenor music may now be omitted without error
- midi instruments may be specified
- midi channel mapping is by instrument
- midi instruments are by voice, not staff
- regression tests added
- documentation minimally updated