ox: Add external footnotes definitions in parse tree
* lisp/ox.el (org-export-get-environment):
(org-export-collect-tree-properties):
Remove :footnote-definition-alist property.
(org-export-get-footnote-definition): Apply removal of property.
(org-export--merge-external-footnote-definitions): New function.
(org-export-as): Use new function.
* testing/lisp/test-ox.el (test-org-export/footnotes): Update tests.
This change allows to have all footnote definitions within the parse
tree, so they can be reached with, e.g., a parse tree filter.