org-export: Define new interface for `org-export-dispatch'
* contrib/lisp/org-export.el (org-export-define-backend):
Add :menu-entry keyword.
(org-export-define-derived-backend): Add :menu-entry
and :sub-menu-entry keywords.
(org-export-dispatch-menu-entries): New variable.
(org-export-dispatch): Define a new interface.
(org-export-dispatch-ui): Rewrite function.
(org-export-dispatch-action): New function.
Every back-end defined through `org-export-define-backend' or
`org-export-define-derived-backend' can specify a menu entry for
`org-export-dispatch'. Navigating the menu is now a two levels
process.