org-src: Allow to edit inline footnote references
* lisp/org-src.el (org-src--edit-element): Do not rely on :value to
extract contents. Small refactoring.
(org-src--contents-area): Renamed from
`org-src--element-contents-area'. Throw an error on unknown
elements.
(org-src--on-datum-p): Rename from `org-src--on-element-p'. Handle
objects.
(org-edit-export-block, org-edit-src-code,
org-edit-fixed-width-region, org-edit-table.el): Apply renaming.
(org-edit-src-save, org-edit-src-exit): Handle inline text.
(org-edit-src-exit): Allow empty or blank code. Handle inline text.
(org-src--edit-element): Rename an argument
(org-edit-footnote-reference): Allow to edit inline definitions.
* etc/ORG-NEWS: Document new feature.