This commit adds two extensions to the OpenDocument writer,
`xrefs_name` and `xrefs_number`.
Links to headings, figures and tables inside the document are
substituted with cross-references that will use the name or caption
of the referenced item for `xrefs_name` or the number for `xrefs_number`.
For the `xrefs_number` to be useful heading numbers must be enabled
in the generated document and table and figure captions must be enabled using for example the `native_numbering` extension.
In order for numbers and reference text to be updated the generated
document must be refreshed.
Co-authored-by: Nils Carlson <nils.carlson@ludd.ltu.se>
