contrib/lisp/org-export: Improve footnote API
* contrib/lisp/org-export.el (org-export-initial-options): Rename
`:footnotes-labels-alist' property into `:footnote-definition-alist'
for consistency. Add comments to code.
(org-export-persistent-properties-list): Apply previous renaming.
(org-export-update-info): Rename `:seen-footnote-labels' into
`footnote-seen-labels'.
(org-export-collect-footnote-definitions,
org-export-footnote-first-reference-p,
org-export-get-footnote-definition,
org-export-get-footnote-number): New functions.