ox: Change back-ends internal representation to structures

ox: Change back-ends internal representation to structures

* lisp/ox.el (org-export--registered-backends): Renamed from
  `org-export-registered-backends'.
(org-export-invisible-backends): Removed variable.
(org-export-get-backend, org-export-get-all-transcoders
org-export-get-all-options, org-export-get-all-filters): New
functions.  It replaces `org-export-backend-translate-table'.
(org-export-barf-if-invalid-backend, org-export-derived-backend-p,
org-export-define-backend, org-export-define-derived-backend):
Rewrite functions using new representation.
(org-export-backend-translate-table): Remove function.
(org-export-get-environment): Use new function.
(org-export--get-subtree-options, org-export--parse-option-keyword,
org-export--get-inbuffer-options, org-export--get-global-options,
org-export-to-buffer org-export-to-file, org-export-string-as
org-export-replace-region-by): Update docstring.
(org-export-data-with-translations): Remove function.  Use
`org-export-data-with-backend' with a temporary back-end instead.
(org-export-data-with-backend, org-export-as): Reflect new definition
for back-ends.
(org-export--dispatch-action, org-export--dispatch-ui): Reflect new
definition for back-ends and variable removal.  Refactoring.
(org-export-filter-apply-functions): Call functions with
current back-end's name, not full back-end.
* lisp/org.el (org-export-backends, org-create-formula--latex-header):
Use new structure and variables.
* testing/lisp/test-ox.el: Update tests.

This patch separates back-end definition from its registration.  Thus,
it allows to use anonymous or unregistered back-ends.