[xliff] Attribute value extensibility

Hi everyone,

I was supposed to post an updated text for the section on Attribute Values
Extensibility, but it seems that we are not yet out of the woods for that
one. Christian and I have looked into various solutions, and asked for help
and suggestions in various places, but it seems (so far) that the only way
to get extension and be able to validate them is using the one already
described or other similar XSD mechanisms with the same drawbacks:
- difficulty to mix different user-defined sets of values.
- need to alter one of the XLIFF XSDs or create a derived one.

Two XSD experts have kindly suggested another methods: have a list of
predefined values and allow for any other NMTOKEN values. They showed us how
to defined that. This would mean (as I understand it so far) that we could
use something like:

<trans-unit id='1' restype='myext:MyValue'></trans-unit>

But we wouldn't be able to validate that 'myext:MyValue' is valid.
I'm also not sure yet wether or not values without namespace prefix but not
in the list would generate errors.
In short: we need to do still more tries/research. We're still waiting for
some feedback, but it doesn't look too promising.

If anyone has ideas... :-(
-yves