[Metadata] fix XSD to match spec

[nigelmegitt]

The metadata XSD does not match the updated specs; it still requires elements in specific orders and for example does not allow multiple ebuttm:appliedProcessing elements.

Update the XSD to match and fix the addition of appliedProcessing wherever it is done.

[EyalLavi]

To clarify: will this effectively result in an XSD for EBU-TT part M?

[nigelmegitt]

That would probably be a good idea!

[EyalLavi]

As discussed yesterday, this will be a bigger piece of work so bumping to a later release.