With this change both legacy language/country elements and IETF BCP 47
language tags will be normalized when chapters are read or
written. This fixes a couple of corner cases in all programs dealing
with chapters:
1. IETF BCP 47 elements will now always be created before writing
chapters unless IETF BCP 47 elements are disabled. This wasn't
always the case when chapters were read from Matroska files.
2. When a chapter display element contains legacy language & country
elements but no IETF BCP 47 elements and IETF BCP 47 elements
aren't disabled, the IETF BCP 47 elements created will contain the
region from the legacy element. Before the change the elements
created didn't contain a country, leading to a change in semantics
as IETF BCP 47 elements take precedence over all legacy elements
when they're present.
3. Legacy country elements are now created when IETF BCP 47 elements
are present & contain a region code allowed in legacy country
elements.
Part of the fix of #3193.
