Add an XSD

[nigelmegitt]

Closes #272 by adding an XSD based on the TTML2 XSD (by copy rather than reference) and making additions and modifications as needed to meet the constraints of DAPT.

With thanks to @magicbadger for his help and content while developing this.

[himorin]

just note, all http://www.w3.org/ns are automatically redirected to https
might not cause error over tooling (per past survey on whole ns redirection), but leave comment here just to note.

[nigelmegitt]

just note, all http://www.w3.org/ns are automatically redirected to https might not cause error over tooling (per past survey on whole ns redirection), but leave comment here just to note.

Are you suggesting that the Namespaces in the spec should be changed to https (which would be a separate issue)? I don't think there's any requirement that anyone or any implementation actually fetches the resource at the URL, and certainly I don't expect the XSD to cause that. I'd just like to know more about what concerns there might be about the https redirect.