The Builder.simpleTable now only adds a row to the TableHead when the
given header row is not null. This uncovered an inconsistency in the
readers: some would unconditionally emit a header filled with empty
cells, even if the header was not present. Now every reader has the
conditional behaviour. Only the XWiki writer depended on the header
row being always present; it now pads its head as necessary.
- Writers.Native is now adapted to the new Table type.
- Inline captions should now be conditionally wrapped in a Plain, not
a Para block.
- The toLegacyTable function now lives in Writers.Shared.
