Xliff: use a real namespace

[srl295]

MF2xliff.ts line 29 has "message format/2.0/not-real-yet" - we have the host schemas.unicode.org reserved for this purpose so i would recommend using that . But really, we should write an xsd for the 'mf' xliff namespace.

[eemeli]

(Sorry for the delay, I've been on vacation the past two weeks)

I agree that a definition of the mf namespace in XLIFF would be Really Good, but at least thus far we've not worked on it in the MessageFormat WG, and it's not clear if it's something that ought to be defined under Unicode, or e.g. in the Oasis XLIFF TC. The implementation here is really a PoC/strawman sort of thing, with no guarantees of stability -- hence the rather explicit not-real-yet part in the namespace.

OTOH, it is a decent idea to define it for its use here, and publish it under this project, to make it easier to work with.