* 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
