ox-latex: Introduce LATEX_HEADER_EXTRA keyword

* lisp/ox-latex.el (latex): Introduce new buffer keyword.
(org-latex-template): Use new keyword.
* lisp/ox-beamer.el (org-beamer-template): Use new keyword.
* lisp/org.el (org-create-formula--latex-header): Use new keyword.
* contrib/lisp/ox-koma-letter.el (org-koma-letter-template): Use new
  keyword.

Previewing of LaTeX snippets takes account of LATEX_HEADER keywords.
LATEX_HEADER_EXTRA keyword allows to specify some header lines that
will not be used for building snippets.

contrib/lisp/ox-koma-letter.el

@@ -194,7 +194,8 @@ holding export options."
              document-class-string
              org-latex-default-packages-alist ; defined in org.el
              org-latex-packages-alist nil     ; defined in org.el
-             (plist-get info :latex-header-extra)))
+	     (concat (plist-get info :latex-header)
+		     (plist-get info :latex-header-extra))))
            info)))))
    ;; Define "From" data.
    (format "\\setkomavar{fromname}{%s}\n"

lisp/org.el

@@ -18159,7 +18159,7 @@ share a good deal of logic."
      (org-combine-plists
       (org-export--get-global-options 'latex)
       (org-export--get-inbuffer-options 'latex))
-     :latex-header-extra))))
+     :latex-header))))
 
 ;; This function borrows from Ganesh Swami's latex2png.el
 (defun org-create-formula-image-with-dvipng (string tofile options buffer)

lisp/ox-beamer.el

@@ -860,7 +860,8 @@ holding export options."
 	       document-class-string
 	       org-latex-default-packages-alist
 	       org-latex-packages-alist nil
-	       (concat (plist-get info :latex-header-extra)
+	       (concat (plist-get info :latex-header)
+		       (plist-get info :latex-header-extra)
 		       (plist-get info :beamer-header-extra))))
 	     info)))))
      ;; 3. Insert themes.

lisp/ox-latex.el

@@ -29,9 +29,10 @@
 ;; functions are available: `org-latex-publish-to-latex' and
 ;; `org-latex-publish-to-pdf'.
 ;;
-;; The library introduces three new buffer keywords: "LATEX_CLASS",
-;; "LATEX_CLASS_OPTIONS" and "LATEX_HEADER", and a new OPTIONS item:
-;; "textht".
+;; The library introduces four new buffer keywords: "LATEX_CLASS",
+;; "LATEX_CLASS_OPTIONS", "LATEX_HEADER" and "LATEX_HEADER_EXTRA" (the
+;; latter will not be used to build LaTeX snippets).  It also
+;; introduces a new OPTIONS item: "textht".
 ;;
 ;; Table export can be controlled with a number of attributes (through
 ;; ATTR_LATEX keyword).
@@ -177,7 +178,8 @@
 		  (:date-format nil nil org-latex-date-timestamp-format)
 		  (:latex-class "LATEX_CLASS" nil org-latex-default-class t)
 		  (:latex-class-options "LATEX_CLASS_OPTIONS" nil nil t)
-		  (:latex-header-extra "LATEX_HEADER" nil nil newline)
+		  (:latex-header "LATEX_HEADER" nil nil newline)
+		  (:latex-header-extra "LATEX_HEADER_EXTRA" nil nil newline)
 		  (:latex-hyperref-p nil "texht" org-latex-with-hyperref t)))
 
 
@@ -1149,7 +1151,8 @@ holding export options."
 	       document-class-string
 	       org-latex-default-packages-alist
 	       org-latex-packages-alist nil
-	       (plist-get info :latex-header-extra)))
+	       (concat (plist-get info :latex-header)
+		       (plist-get info :latex-header-extra))))
 	     info)))))
      ;; Possibly limit depth for headline numbering.
      (let ((sec-num (plist-get info :section-numbers)))