org-export: Internal changes to back-end definition

org-export: Internal changes to back-end definition

* contrib/lisp/org-export.el (org-export-registered-backends): New
  variable.
(org-export-define-backend, org-export-define-derived-backend): Use
new variable. Also redefine how sub-menus are defined.
(org-export-backend-filters, org-export-backend-menu,
org-export-backend-options, org-export-backend-translate-table): New
functions.
(org-export-get-environment, org-export--parse-option-keyword,
org-export--get-subtree-options, org-export--get-inbuffer-options,
org-export--get-global-options, org-export-install-filters,
org-export-with-backend): Access to data stored in new variable.
(org-export-dispatch-ui): Display sub-menus according to new
definition.
(org-export-dispatch-menu-entries): Removed variable.
* contrib/lisp/org-e-beamer.el: Use new sub-menu definition.
(org-e-beamer--format-section, org-e-beamer-item,
org-e-beamer-keyword): Use `org-export-with-backend' instead of
relying on removed variables.
* testing/lisp/test-org-export.el: Update tests.

This patch gets rid of "invisible" variables, that is variables
defvar'ed within a macro.