Specification: Add XML catalogues for released schemas

This should allow future simplification of our entity resolver and ease maintenance, since future schema updates will only require a tweak to the XML catalogues rather than editing code.  This will also be usable by the C++ implementation, and will also permit the schemas to be resolvable both in the build and install trees.

Things not done in this PR but which would be useful to add:

- Package the catalogues in the jar alongside the xsd files
- Use a catalogue entity resolver to replace the existing hardcoded entries in the entity resolver with the toplevel catalog.xml which should bring in all catalogued schemas
- Install alongside the xsd files in the C++ implementation
- Use a custom catalogue entity resolver in the C++ implementation

I'll be able to do the latter two C++ changes on the currently open PRs are merged.

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Testing:

- Do the xml.xsd and XMLschema.xsd files still get packaged and work in their new location?
- Are all the catalogue entries valid (correct URL and filename)?
- Any missing catalogue entries?

/cc @qidane 

--no-rebase