Writer.Docbook improvements: tight and loose lists, section levels

Current Docbook output renders tight and loose lists identically. By default, Docbook lists are loose, but it provides an attribute `spacing="compact"` to specify a tight list, on the list element itself. This commit adds the attribute when rendering tight lists. A list is considered to be tight if the first block of the first item is Plain.

Docbook can number section levels explicitly, eg `<sect1>`, or it can use the generic `<section>` element and let the nesting determine the level. The latter is preferable when Pandoc output needs to be inserted as a fragment into a larger Docbook document that already contains sections, because the fragment can then fit into any level in the section hierarchy.