Move org-element commands to org.el and rename them.

Move org-element commands to org.el and rename them.

* org.el (org-forward-element): Rename from
`org-element-forward'.
(org-backward-element): Rename from `org-element-backward'.
(org-up-element): Rename from `org-element-up'.
(org-down-element): Rename from `org-element-down'.
(org-drag-element-backward): Rename from
`org-element-drag-backward'.
(org-drag-element-forward): Rename from
`org-element-drag-forward'.
(org-mark-element): Rename from `org-element-mark-element'.
(org-transpose-element): Rename from `org-element-transpose'.
(org-unindent-buffer): Rename from
`org-element-unindent-buffer'.
(org-mode-map): Update the names of a commands.
Remove useless declarations.

* org-element.el (org-element-forward, org-element-backward)
(org-element-up, org-element-down)
(org-element-drag-backward, org-element-drag-forward)
(org-element-mark-element, org-narrow-to-element)
(org-element-transpose, org-element-unindent-buffer): Move to
org.el.