Doxia XDOC parser and XHTML renderer ignore "rowspan" and "colspan" attributes for tables

The xdoc parser ignores rowspan and colspan attributes for <th> and <td> cells (even though the DTD says that these are legal) and the XHTML renderer subsequently does not render out these attributes.

As the Sink-API offers methods for generating table-cells with a defined width, this is easy to implement (see attached patch), but the rowspan attribute would require new methods in the (very unflexible) API that take both height and width.

The attached patch fixes the colspan problem for the xdoc parser and the xhtml renderer.