Improve handling of hard line breaks in Docbook writer

 * Use a <literallayout> for the entire paragraph, not just for the
   newline character
 * Don't let LineBreaks inside footnotes influence the enclosing
   paragraph
Add some unit tests for Writers.Docbook

These are primarily aimed at testing the new treatment of line breaks,
but hopefully other tests can be added more easily now as features
and changes are implemented in the writer.

Adapted from Tests.Writers.HTML.tests.