Update content models w.r.t. permissible styling attributes (#1155).

[skynavga]

Closes #1155.

This PR is untestable for the same reasons described at #1096 (comment).

[nigelmegitt]

I think this is in fact testable, since it is constraining the set of styling attributes that are permitted. Therefore an invalidity test can be constructed that includes an animate or set element (or both) that targets tts:ruby, for example.

[skynavga]

@nigelmegitt yes, that is true, but only for the changes to the content models of animate and set; however, I planned to add tests for that constrain (can't target non-animatable styles) in the context of #1154, which adds other language proscribing this usage without needing the change in content model to do so

[nigelmegitt]

@skynavga okay, that would work too.

[nigelmegitt]

Assuming the only change that needs reviewing here is e8223bd, since this also merges in changes from #1154, this looks good to me.