[xliff] Extensibility

Hello everyone,

Thanks to Christian's help, we have now an example that shouldn't go athwart
like the previous version. The attached zip file contains all the files you
need to try it. Some of them have been updated: some commented lined needed
to be uncommented in the core XSD, and one element name (appinfo) was
misspelled (appInfo) in the value XSD and its corresponding style sheet.

Cheers,
-yves