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

etc/generate-fixtures

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 for org_file in org/testdata/*.org; do
+    echo $org_file
     ./go-org $org_file html > org/testdata/$(basename $org_file .org).html
     ./go-org $org_file org > org/testdata/$(basename $org_file .org).pretty_org
 done