The same generated index the table of contents is, over the captions of one
category instead of the headings — an `index` attr picks the family, so the
node view, the page numbers and the leader dots are shared. Round-trips as
ODF text:illustration-index / text:table-index, found by the counter's name
the way LibreOffice finds them, and Word's TOC \c "Figure" field.
Steffen Becker
f455a7c82f