Change "feature designator" throughout to "extension designation". #7
Description
Feature designations (not designators) can only be defined in the core TTML specifications, i.e., TTML1, TTML2, etc. Extension modules published by the TTWG should use extension designations, about which see [TTML2] Appendix F.1 [1].
[1] https://www.w3.org/TR/2018/REC-ttml2-20181108/#extension-designations
Marking this substantive since the designation namespace needs to shift from the TT Feature Namespace (http://www.w3.org/ns/ttml/feature/) to the TT Extension Namespace (http://www.w3.org/ns/ttml/extension/). N.B. at present the language defining #karaoke is not sufficiently precise to determine which namespace applies.