org-element: Interpret secondary strings with `org-element-interpret-data'
* contrib/lisp/org-element.el (org-element-headline-interpreter,
org-element-inlinetask-interpreter, org-element-item-interpreter,
org-element-footnote-reference-interpreter,
org-element-interpret--affiliated-keywords): Use
`org-element-interpret-data' instead of
`org-element-interpret-secondary'.
(org-element-interpret-data): Handle secondary strings. Refactor
code.
(org-element-interpret-secondary): Remove function
* contrib/lisp/org-export.el (org-export-table-cell-alignment): Use
`org-element-interpret-data' instead of
`org-element-interpret-secondary'.