Different media/tag formats specify languages and countries
differently. This change introduces a Locale class to keep track
of the format being used. So far there are no automatic conversions
implemented so it is entirely up to the user to pass valid values using
a format which matches the one required by the media/tag format.
This change also adds support for Matroska's IETF elements so at least the
raw value can be read, written and is preserved.
For now let's just ignore these elements explicitely until they are
actually supported. This way the warnings are at least more specific and
there will be no inconsistency when updating the track language.
Martchus