schema: ODF 1.3 + libreoffice extensions schema

Build this with RelaxNG <include> feature instead of a complete schema.

This isn't bad but could work better if the schema were a little more
modular...

In order to track down all the uses of extension namespaces like loext,
invoke the validator without "-e".

Change-Id: I92a666fc54c25d66cea38decdc188d60de2a0b74
Reviewed-on: https://gerrit.libreoffice.org/56488
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

test/source/bootstrapfixture.cxx

@@ -169,7 +169,9 @@ void test::BootstrapFixture::validate(const OUString& rPath, test::ValidationFor
 
     if (eFormat == test::ODF)
     {
-        aValidator += " -e -M "
+        // invoke without -e so that we know when something new is written
+        // in loext namespace that isn't yet in the custom schema
+        aValidator += " -M "
             + m_directories.getPathFromSrc("/schema/libreoffice/OpenDocument-manifest-schema-v1.3+libreoffice.rng")
             + " -D "
             + m_directories.getPathFromSrc("/schema/libreoffice/OpenDocument-dsig-schema-v1.3+libreoffice.rng")