Set context-id for implicit voices in polyphony shorthand.

Set context-id for implicit voices in polyphony shorthand.

Since voicify-music now uses the context alias `Bottom' to create the
appropriate voice-level contexts in a << { } \\ { } >> construct, the
context-id sent via context-spec-music is ignored.

* add context-id as arg to Global_context/Context::get_default_interpreter (),
  with empty string as default

* pass context-id to Context::get_default_interpreter () so it can
  be used in call to create_context () instead of empty string

* add regtest, which will fail to compile if context-id is invalid