In rST, the "figure" directive's content must start with a paragraph (that
becomes the caption) or an empty comment (to get a legend without caption).
The change allows passing a "classes" or "names" attribute value to the
figure caption via the "class" directive rsp. an internal hyperlink target.
Change the `references.PropagateTargets` transform to remove a `<target>`
element followed by a `<caption>` (after propagating ids and names)
to keep the doctree valid.
Fix the HTML5 writer, to apply the attributes to the `<figcaption>` element,
(instead of the included `<p>`).
Fix LaTeX writer to handle "classes" and "ids" attributes of a `<caption>`
element.
Add test cases.
Remove misleading test case. Figures without caption are invalid and won't get
a "Figure 1."-style caption with the Docutils writers.
