Update a few keybindings in org.el and fix a problem in org-element.el.

* org.el: Update a few keybindings.

Use [(control down)] and "\M-}" for `org-element-forward'.
Use [(control up)] and "\M-{ `org-element-backward'.
Use "\C-c\C-^" for `org-element-up'.
Use "\C-c\C-_" for `org-element-down'.
Use "\C-c\C-@" for `org-element-mark-element'.

* org-element.el (org-element-down): Throw an error when the
element has no content.

lisp/org-element.el

@@ -4365,7 +4365,8 @@ Move to the previous element at the same level, when possible."
      ((memq (org-element-type element) org-element-greater-elements)
       ;; If contents are hidden, first disclose them.
       (when (org-element-property :hiddenp element) (org-cycle))
-      (goto-char (org-element-property :contents-begin element)))
+      (goto-char (or (org-element-property :contents-begin element)
+		     (error "No content for this element"))))
      (t (error "No inner element")))))
 
 (defun org-element-drag-backward ()

lisp/org.el

@@ -17730,7 +17730,7 @@ BEG and END default to the buffer boundaries."
 ;; | `outline-next-visible-heading'     | `C-c C-n'   | still same function   |
 ;; | `outline-previous-visible-heading' | `C-c C-p'   | still same function   |
 ;; | `outline-up-heading'               | `C-c C-u'   | still same function   |
-;; | `outline-move-subtree-up'          | `C-c C-^'   | better: org-shiftup   |
+;; | `outline-move-subtree-up'          | overridden  | better: org-shiftup   |
 ;; | `outline-move-subtree-down'        | overridden  | better: org-shiftdown |
 ;; | `show-entry'                       | overridden  | no replacement        |
 ;; | `show-children'                    | `C-c C-i'   | visibility cycling    |
@@ -17838,14 +17838,16 @@ BEG and END default to the buffer boundaries."
 (if (boundp 'narrow-map)
     (org-defkey narrow-map "e" 'org-narrow-to-element)
   (org-defkey org-mode-map "\C-xne" 'org-narrow-to-element))
+(org-defkey org-mode-map "\C-\M-t"  'org-element-transpose)
+(org-defkey org-mode-map [(control down)]  'org-element-forward)
+(org-defkey org-mode-map [(control up)]    'org-element-backward)
+(org-defkey org-mode-map "\M-}"    'org-element-forward)
+(org-defkey org-mode-map "\M-{"    'org-element-backward)
+(org-defkey org-mode-map "\C-c\C-^"   'org-element-up)
+(org-defkey org-mode-map "\C-c\C-_"   'org-element-down)
 (org-defkey org-mode-map "\C-\M-t"    'org-element-transpose)
-(org-defkey org-mode-map "\C-\M-f"    'org-element-forward)
-(org-defkey org-mode-map "\C-\M-b"    'org-element-backward)
-(org-defkey org-mode-map "\C-\M-u"    'org-element-up)
-(org-defkey org-mode-map "\C-\M-d"    'org-element-down)
-(org-defkey org-mode-map "\C-\M-t"    'org-element-transpose)
-(org-defkey org-mode-map "\C-c\C-f"    'org-forward-same-level)
-(org-defkey org-mode-map "\C-c\C-b"    'org-backward-same-level)
+(org-defkey org-mode-map "\C-c\C-f"   'org-forward-same-level)
+(org-defkey org-mode-map "\C-c\C-b"   'org-backward-same-level)
 (org-defkey org-mode-map "\C-c$"    'org-archive-subtree)
 (org-defkey org-mode-map "\C-c\C-x\C-s" 'org-advertized-archive-subtree)
 (org-defkey org-mode-map "\C-c\C-x\C-a" 'org-archive-subtree-default)
@@ -17915,7 +17917,7 @@ BEG and END default to the buffer boundaries."
 (org-defkey org-mode-map "\C-c\C-x\C-mg"    'org-mobile-pull)
 (org-defkey org-mode-map "\C-c\C-x\C-mp"    'org-mobile-push)
 (org-defkey org-mode-map "\C-c@" 'org-mark-subtree)
-(org-defkey org-mode-map "\C-c\C-@" 'org-mark-list)
+(org-defkey org-mode-map "\C-c\C-@" 'org-element-mark-element)
 (org-defkey org-mode-map [?\C-c (control ?*)] 'org-list-make-subtree)
 ;;(org-defkey org-mode-map [?\C-c (control ?-)] 'org-list-make-list-from-subtree)