Fix xrefs to preface topics #1732

The "commonattributes" template wasn't called for preface topics, which
meant that no @id attribute was added for them when generating the
output XSL-FO markup.
Merge pull request #1733 from eerohele/issues/1732

Fix xrefs to preface topics #1732
Clean chunking code
Fix namespaces attribute writing with XML utils #1728
Fix onlytopic.in.map documentation #1488