Improve handling of image subtitle IDs in XML (DoM bug #1965)

Improve handling of image subtitle IDs in XML (DoM bug #1965)

When reading/writing the XML for image subtitles, we assumed that
the content of the <Image> tag is just the ID of the PNG in the MXF,
without any prefix.

DoM bug #1965 mentions a DCP where this is not the case, and SMPTE
429-5-2009 has an example where there is urn:uuid: in the XML.

This change makes DoM write this urn:uuid: prefix, and accept it if
it's present (but not complain if it's not).

If the urn:uuid: _is_ required in the field, it's a bit surprising
that nobody has complained up to this point.  Maybe nobody noticed,
or nobody reported it.