Let check_local_options handle nested curly braces

[uliska]

Nested curly braces in an optional argument break the existing code, if a closing brace happens to be followed by a comma, like in

caption={Score with \emph{two}, three or four bars}

This commit keeps track of the nesting level to ensure the whole argument is stored as a single option value. (Faulty input will result in follow-up errors, but these should be clear from the LaTeX error messages).

[jperon]

@uliska Could you please check it works with last commit ? If it does, please "squash and merge" it.

[uliska]

Yes it does. Thank you for the concise solution. I did in fact doubt my solution was the best implementation.