Write braces around macros only when actually needed

Write braces around macros only when actually needed

When a macro with optionals appeared inside the optional argument of
another one, the onscreen display and latex output were wrong. This
issue was addressed at [e8f480e7/lyxgit] by enclosing in braces macros
with optional arguments. However, this was done even when the macro
with optionals was in a non-optional argument of another macro.
This commit limits the bracing to the cases where it is really needed
and allows to address some particular issues evidenced in #11552.