org-element: Syntax change for caption

org-element: Syntax change for caption

* contrib/lisp/org-element.el (org-element-dual-keywords): Move
  caption in the dual keywords category.
(org-element--affiliated-re): Change regexp matching a dual keyword.
(org-element-collect-affiliated-keywords): Dual parsed keywords also
  get their optional value parsed as a secondary string.
* EXPERIMENTAL/org-e-latex.el (org-e-latex--caption/label-string,
  org-e-latex-src-block): Apply caption status change.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii--build-caption): Apply
  caption status change.
(org-e-ascii--list-listings, org-e-ascii--list-tables): Apply caption
  status change.  Also use short caption name when available.

The status change implies a syntax change.  Captions can now share
#+results: syntax, which mean that they can have a secondary
value. Thus, the following Org snippets are valid, the first two being
equivalent.

#+caption: long name
#+caption[]: long name
#+caption[short name]: long name