* lily/translator-def.cc (get_translator_names): new function

* lily/translator-def.cc (get_translator_names): new function
(add_context_mod): new function
(get_accepted): new function

* lily/parser.yy (context_mod): put all modifications of
\translator { } in a single production, encode as SCM.

* lily/translator-def.cc (add_context_mod): rewrite Translator_def
to use read-only lists, so additions to the definition can be
prepended easily.

* Documentation/user/invoking.itexi (Editor support): add node on
editor support.

* Documentation/user/refman.itely (Bar check): add lyrics example
of bar checks.

* input/mutopia/F.Schubert/morgenlied.ly: add bar checks.

* scripts/convert-ly.py (conv): add conversion rule.

* lily/parser.yy: use \change iso. \translator for staff switch.

* lily/my-lily-lexer.cc: add \change.

* scm/define-translator-properties.scm
(default-melisma-properties): add definition.