ox: Introduce "raw" pseudo objects

* lisp/ox.el (org-export-raw-string): New function
(org-export-data):
(org-export-with-backend): React to raw objects.
* testing/lisp/test-ox.el (test-org-export/raw-string): New test.

A raw object is a pseudo-object (i.e., special object type that exists
only during export) with the property of being exported as-is, with no
processing from an export back-end.

It is particularly useful to add contents to, or pre-process objects
from, a parse tree.

lisp/ox.el

@@ -1881,6 +1881,8 @@ Return a string."
 		(cond
 		 ;; Ignored element/object.
 		 ((memq data (plist-get info :ignore-list)) nil)
+                 ;; Raw code.
+                 ((eq type 'raw) (car (org-element-contents data)))
 		 ;; Plain text.
 		 ((eq type 'plain-text)
 		  (org-export-filter-apply-functions
@@ -1947,7 +1949,7 @@ Return a string."
 	   data
 	   (cond
 	    ((not results) "")
-	    ((memq type '(org-data plain-text nil)) results)
+	    ((memq type '(nil org-data plain-text raw)) results)
 	    ;; Append the same white space between elements or objects
 	    ;; as in the original buffer, and call appropriate filters.
 	    (t
@@ -3668,7 +3670,8 @@ the communication channel used for export, as a plist."
   (when (symbolp backend) (setq backend (org-export-get-backend backend)))
   (org-export-barf-if-invalid-backend backend)
   (let ((type (org-element-type data)))
-    (when (memq type '(nil org-data)) (error "No foreign transcoder available"))
+    (when (memq type '(nil org-data raw))
+      (error "No foreign transcoder available"))
     (let* ((all-transcoders (org-export-get-all-transcoders backend))
 	   (transcoder (cdr (assq type all-transcoders))))
       (unless (functionp transcoder) (error "No foreign transcoder available"))
@@ -4562,6 +4565,17 @@ objects of the same type."
 	    ((funcall predicate el info) (cl-incf counter) nil)))
 	 info 'first-match)))))
 
+;;;; For Raw objects
+;;
+;; `org-export-raw-string' builds a pseudo-object out of a string
+;; that any export back-end returns as-is.
+
+(defun org-export-raw-string (s)
+  "Return a raw object containing string S.
+A raw string is exported as-is, with no additional processing
+from the export back-end."
+  (unless (stringp s) (error "Wrong raw contents type: %S" s))
+  (org-element-create 'raw nil s))
 
 ;;;; For Src-Blocks
 ;;

testing/lisp/test-ox.el

@@ -3811,6 +3811,17 @@ Another text. (ref:text)
 	    (org-element-adopt-elements paragraph "begin " object "end")
 	    (org-export-data-with-backend paragraph backend nil)))))
 
+
+;;; Raw objects
+
+(ert-deftest test-org-export/raw-strings ()
+  "Test exporting raw objects."
+  (should
+   (equal "foo"
+          (let ((backend (org-export-create-backend))
+                (object (org-export-raw-string "foo")))
+            (org-export-data-with-backend object backend nil)))))
+
 
 ;;; Src-block and example-block