Add `:caption' attribute to #+ATTR_LATEX property

* lisp/ox-latex.el (org-latex--caption/label-string): Add ability,
which can build a caption string from `:caption' attribute of #+ATTR_LATEX.
(org-latex--inline-image,org-latex--org-table): Tiny change.
* doc/org.texi (@LaTeX{} specific attributes): Document `:caption'
attribute of #+ATTR_LATEX.

This feature is very useful when you export org to latex with custom
caption command, for example:

\#+ATTR_LATEX: :caption \BiTableCaption{caption 1}{caption 2}
|---+---|
| x | y |
|---+---|
| 1 | 2 |
|---+---|

doc/org.texi

@@ -11599,6 +11599,11 @@ Environment used for the table.  It can be set to any @LaTeX{} table
 environment, like @code{tabularx}, @code{longtable}, @code{array},
 @code{tabu}, @code{bmatrix}@enddots{}  It defaults to
 @code{org-latex-default-table-environment} value.
+@item :caption
+@code{#+CAPTION} keyword is the simplest way to set a caption for a table
+(@pxref{Images and tables}).  If you need more advanced commands for that
+task, you can use @code{:caption} attribute instead.  Its value should be raw
+@LaTeX{} code.  It has precedence over @code{#+CAPTION}.
 @item :float
 @itemx :placement
 Float environment for the table.  Possible values are @code{sidewaystable},
@@ -11651,6 +11656,16 @@ a table that will span over multiple pages, or a matrix product:
 | 3 | 4 |
 @end example
 
+In the example below, @LaTeX{} command
+@code{\bicaption@{HeadingA@}@{HeadingB@}} will set the caption.
+
+@example
+#+ATTR_LATEX: :caption \bicaption@{HeadingA@}@{HeadingB@}
+| ..... | ..... |
+| ..... | ..... |
+@end example
+
+
 @subsubheading Images in @LaTeX{} export
 @cindex images, inline in @LaTeX{}
 @cindex inlining images in @LaTeX{}
@@ -11672,6 +11687,14 @@ example:
 [[./img/sed-hr4049.pdf]]
 @end example
 
+If you need a specific command for the caption, use @code{:caption}
+attribute.  It will override standard @code{#+CAPTION} value, if any.
+
+@example
+#+ATTR_LATEX: :caption \bicaption@{HeadingA@}@{HeadingB@}
+[[./img/sed-hr4049.pdf]]
+@end example
+
 If you have specified a caption as described in @ref{Images and tables}, the
 picture will be wrapped into a @code{figure} environment and thus become
 a floating element.  You can also ask Org to export an image as a float
@@ -11768,6 +11791,17 @@ Therefore, any even number greater than 2 is the sum of two primes.
 \end@{proof@}
 @end example
 
+If you need to insert a specific caption command, use @code{:caption}
+attribute.  It will override standard @code{#+CAPTION} value, if any.  For
+example:
+
+@example
+#+ATTR_LATEX: :caption \MyCaption@{HeadingA@}
+#+BEGIN_PROOF
+...
+#+END_PROOF
+@end example
+
 @subsubheading Horizontal rules
 @cindex horizontal rules, in @LaTeX{} export
 

lisp/ox-latex.el

@@ -881,8 +881,11 @@ For non-floats, see `org-latex--wrap-label'."
 		      (format "\\label{%s}"
 			      (org-export-solidify-link-text label))))
 	 (main (org-export-get-caption element))
-	 (short (org-export-get-caption element t)))
+	 (short (org-export-get-caption element t))
+	 (caption-from-attr-latex (org-export-read-attribute :attr_latex element :caption)))
     (cond
+     ((org-string-nw-p caption-from-attr-latex)
+      (concat caption-from-attr-latex "\n"))
      ((and (not main) (equal label-str "")) "")
      ((not main) (concat label-str "\n"))
      ;; Option caption format with short name.
@@ -1655,7 +1658,9 @@ used as a communication channel."
 		  (cond ((and (not float) (plist-member attr :float)) nil)
 			((string= float "wrap") 'wrap)
 			((string= float "multicolumn") 'multicolumn)
-			((or float (org-element-property :caption parent))
+			((or float
+			     (org-element-property :caption parent)
+			     (org-string-nw-p (plist-get attr :caption)))
 			 'figure))))
 	 (placement
 	  (let ((place (plist-get attr :placement)))
@@ -2333,7 +2338,9 @@ This function assumes TABLE has `org' as its `:type' property and
 			 ((and (not float) (plist-member attr :float)) nil)
 			 ((string= float "sidewaystable") "sidewaystable")
 			 ((string= float "multicolumn") "table*")
-			 ((or float (org-element-property :caption table))
+			 ((or float
+			      (org-element-property :caption table)
+			      (org-string-nw-p (plist-get attr :caption)))
 			  "table")))))
 	 ;; Extract others display options.
 	 (fontsize (let ((font (plist-get attr :font)))