Add schematron that validates features that Relax NG cannot

The Relax NG schema for ODF cannot check many of the contraints that the specification places on ODF documents. Specifically, the restriction that references to e.g. style names should have corresponding definitions.

Write schematron that checks keys and references and embed it to the Relax NG. The schematron should cover all keys and references that are mentioned in the specification. Ideally, it could check across files so that references in content.xml are checked for in styles.xml