(1) ensure TTML10 schema does not permit #PCDATA in metadata element content;
(2) ensure TTML10 semantic validator doesn't fail on foreign metadata elements;
(3) add (valid) test for foreign metadata information (elements and attributes);
(4) add (invalid) test for #PCDATA in metadata element content;
(5) regenerate JAXB bindings after TTML10 schema change;
(6) update MD documentation;