Improve footnote handling

- Footnotes separator rather than headline to get around i18n
- Warn on footnote redefinition
- Do not export footnote definitions at point of definition, only in the
  footnote section.
- Do not automatically exclude Footnotes section to get around possibly hiding
  other content of such a section - and i18n.
  The user has the choice of explicitly hiding the section via a :noexport:
  tag.

and some other refactoring

org/inline.go

@@ -157,7 +157,7 @@ func (d *Document) parseFootnoteReference(input string, start int) (int, Node) {
 		link := FootnoteLink{name, nil}
 		if definition != "" {
 			link.Definition = &FootnoteDefinition{name, []Node{Paragraph{d.parseInline(definition)}}, true}
-			d.Footnotes.add(name, link.Definition)
+			d.addFootnote(name, link.Definition)
 		}
 		return len(m[0]), link
 	}