Move add-grob-definition from a snippet to scm/translation-functions.scm (issue 6128048)

[dak]

It is not the "official" way to create a new grob definition, but a hack
from Mike done without understanding the implementation of
\grobdefinitions.  It is not just tampering with a global variable
(session-wide instead of parser-wide) to add stuff to
all-grob-descriptions, it is also inefficient and unnecessary since
all-grob-descriptions _have_ already been pulled into the Score context
definition _before_ the user code is even started.

Instead, one can just add a single context mod adding the new grob
definition to the Score context definition.

We really don't need to pull code with bad side effects from a snippet
into the main code.

It would make more sense to revert the respective commit creating the
snippet completely.

http://codereview.appspot.com/6128048/