Issue 5309: Refactor Context::get_score_context ()

https://sourceforge.net/p/testlilyissues/issues/5309/

Global_context::get_score_context () returns the first child, as
before, but it is no longer a virtual method of Context.

Translator::find_score_context () finds the Score context enclosing
the Translator in a way similar to (but slightly more direct than)
\set Score.whatever = ... rather than depending on the global context.

[Dan Eble, 2018-04-25 22:40:26]

On 2018/04/21 18:40:18, Dan Eble wrote:

This will deserve some revision based on questions to be answered in the review
for issue 5310.

[Dan Eble, 2020-01-03 16:08:22]

On 2018/04/25 22:40:26, Dan Eble wrote:
> On 2018/04/21 18:40:18, Dan Eble wrote:
> 
> This will deserve some revision based on questions to be answered in the
review
> for issue 5310.

See updated and expanded changes in
https://codereview.appspot.com/561290043