texi2html: use @subsubheading for interface property headings in the IR

texi2html: use @subsubheading for interface property headings in the IR

Adjust the .css file so subsubheadings look like unnumberedsubsubsec.
The problem with @unnumberedsubsubsec is that it (1) generates an entry
in the TOC and (2) messes up the internal navigation buttons, since they
don't have their own @node, so texi2html uses #SECxx as anchor, which does
not exist...