<xref> elements can also be used to link to <figure> elements. Alas, the
DocBook reader was not aware of this and thus generated a link text
which just said 'figure_title'.
https://tdg.docbook.org/tdg/4.5/figure.html explains that <figure>
elements can contain <title> elements, so let's try to use that if it is
available.
Update tests.
Reason: it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'. This makes it unsuitable for our
purposes.
pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.
Previously we used our own homespun formatting. But this
produces over-long lines that aren't ideal for diffs in tests.
Easier to use something off-the-shelf and standard.
Closes#7580.
Performance is slower by about a factor of 10, but this isn't
really a problem because native isn't suitable as a serialization
format. (For serialization you should use json, because the reader
is so much faster than native.)
Now we properly parse title and subtitle elements that are
direct children of book and article (as well as children of
bookinfo, articleinfo, or info).
We also now use the "subtitle" metadata field for subtitles,
rather than tacking the subtitle on to the title.
