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/util.go

@@ -11,11 +11,6 @@ func isSecondBlankLine(d *Document, i int) bool {
 	return false
 }
 
-func isEmptyLineParagraph(n Node) bool {
-	p, ok := n.(Paragraph)
-	return ok && len(p.Children) == 0
-}
-
 func isImageOrVideoLink(n Node) bool {
 	if l, ok := n.(RegularLink); ok && l.Kind() == "video" || l.Kind() == "image" {
 		return true