* lily/include/grob-info.hh: origin_contexts() now does not

* lily/include/grob-info.hh: origin_contexts() now does not
include the originating engraver

* lily/translator-group.cc (translator_each): clean up contents,
remove superfluous stubs. Engraver_group_engraver and
Performer_group_performer are the only classes using multiple
inheritance.

* lily/include/score-context.hh (class Score_context): interface
between Score_{engraver,performer} and Global_context.

* lily/include/global-context.hh (Context): change from
global-translator. This is the entry point for contexts; it is not
tweakable using \translator{}. Score is no longer the top context,
but sits underneath Global.

* lily/include/context.hh (class Context): brain surgery: split
out Context functionality from Translator_group. Context is now a
separate type from Translator.