scribble: improve HTML handling of 'unnumbered and 'toc-hidden

A part with style property 'toc-hidden no longer
hides child sections, which makes it consistent with Latex/PDF
rendering.

A part with style 'grouper and 'unnumbered does not make its
child parts render as more nested, which is consistent with
'gruper without 'unnumbered. An unnumbered grouper is represented
as "" in a section-number list (while #f is still used for
unnumbered non-grouper layers).
scribble Latex: support all 2d-cond characters