Add xml/validators to checkstyle and do checkstyle cleanup

Add checkstyle checking for xml/validators. Clean up xml/validators from the checkstyle complaints. Update the various validator classes so they are implemented in parallel and not each in a different way. Improve complaints when validation fails. Update schema doc.

And incidentally add the pattern facet to boolean.

This is in preparation to fix a couple pending bugs (or missing features) in the validation area.