Issue 1375/2: Create Translator_creator class

Issue 1375/2: Create Translator_creator class

Previously, translators were created by copying from a context-less
instantiation of the translator containing its documentation.  This had
several unpleasant consequences, the most problematic likely being the
inability to register Scheme engravers because their documentation would
be identical to all other Scheme engravers.

A new Translator_creator class takes over the task of creating
Translator instances when called with a context argument.

As a result of joining the mechanisms for Scheme engravers and C++
engravers, ly:translator-name and ly:translator-description are
reimplemented in a manner resembling object properties.