XMI12 — propose a new tag to be applied to metamodels

If we haven't already I think we need to propose a new tag to be applied to
metamodels which will contain the URI for the namespace (as opposed to the
namespace name which is given by the existing tag xmi.namespace).
e.g. xmi.namespaceURI

Two main reasons for doing this:
a) automated generation of the DTD/schema for standards
b) (and more importantly) allow instance documents to be generated with the
right URI (which is obviously important) - without implementations having to
hardcode a value or guess (which is what they have to do currently).