[xliff] Embedded XLIFF

To follow up on the discussion about embedded XLIFF, here is a basic
proposal for requirements (not a proposal for the actual tags).

It's directly inspired from the one created by ITS, but focused on the
localization directives aspects (ITS's requirement was broader). I've also
tried to have various examples (with different possible notations) to
illustrate how this could be used. The mapping to XLIFF constructs is
usually easy.

Note that I've put all requirements I could found, even if some are resolved
already (like language identification with xml:lang).

cheers
-yves