org-element: Interpret pseudo elements and objects
* lisp/org-element.el (org-element-interpret-data): Change signature.
One can now define pseudo elements and objects types.
(org-element--interpret-data-1): New function.
* testing/lisp/test-org-element.el: Add tests.
* testing/lisp/test-ox.el: Update tests.
A pseudo element or object is a new element or object type that is
created and treated locally within an export back-end. The back-end
provides a translator for it and it is ignored when interpreted back
into Org syntax.