Improve/adjust note/sectioning elements

- Notes are now `<section>`s, not `<div>`s.

- A new “abstract” section type has been added.

- Sections can now contain list items (with a level > 1).

- The `LesML:finalize-footnotes` stage has been repurposed into a more
  general `LesML:finalize` stage, in part to facilitate the above.

- Paragraph i·d¦s are now passed upwards to the containing element in
  all cases, when applied to the first paragraph in a container. To
  literally assign an i·d to the first paragraph, add an empty initial
  paragraph (just a `¶`), which will be dropped. The same is true for
  language tags.

It would be nice to be able to specify titles/labels for sections, but
there isn¦t a good mechanism for doing this as of now. The heading
sigils are probably inappropriate for this, as headings in sections are
conceptually singular and “unleveled”. There is no need for four
heading types when a section can only contain one heading and it must
always be the first paragraph.

But on the H·T·M·L side, section headings _should_ have appropriate
levels corresponding to their position in the document. So this would
need to be remediated somehow.