footer navigation headers

[Per Bothner]

[Sorry to throw all this on you guys all at once, but I'm on a roll,
and it would be nice to fix as many annoyances before the next release
as we can.  Thanks for your help and patience!]

I have two related problems relating to navigation headers that
are not at the top of a page - i.e. "internal" and "at-end".
I'd like to remove the non-initial non-final headers.
Assuming --split=section, I'd like to remove all the headers
generated for @subsection and @subsubsection.  While I agree they
may have some value, I think the extra "clutter" makes them a net
negative, especially when we have the js-info sidebar for navigation.

We can do this with CSS:
    div.subsection div.header { display: none }

However, this has the downside that it also hides the end-of-page
navigation header, which I think is useful.

The problem is that the  end-of-page header is emitted inside the
final @subsection.  I think it should be emitted outside the
final @subsection.

A related problem is that the end-of-page header is relative to the
final @subsection (and duplicates that internal header at the start
of the @subsection).  In my opinion, the header should point to
the next/previous/up of the @section (or @chapter for a chapter
without sections).  This is more logical, I think, whether or not
the @subsecton navigation headers are displayed.

The attached patch is a kludgy proof-of-concept, and assumes
SECTION_FOOTER_BUTTONS=0.  I'm not sure what configuration options
make sense, and what should be the defaults, but combination of
the patch, plus setting SECTION_FOOTER_BUTTONS=0, plus the above CSS rule
seems to work for me.  I also think the result HTML output makes
a good default, and it makes it easy to use CSS to hide the internal headers,
depending on individual preference.
--
        --Per Bothner
per@bothner.com   http://per.bothner.com/

footer-navigation1.patch
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