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.