org-export: Export snippets are not skipped automatically: back-ends decide

org-export: Export snippets are not skipped automatically: back-ends decide

* contrib/lisp/org-export.el (org-export-snippet-backend): New
  function.
(org-export--skip-p): Remove automatic skip of export snippets.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-html.el (org-e-html-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-export-snippet): Use new
  function.
* EXPERIMENTAL/org-e-odt.el (org-e-odt-export-snippet): Use new
  function.
* testing/contrib/lisp/test-org-export.el: Add test.

The idea behind this change is that a given back-end may decide to
keep an export snippet not directly targetted at it (i.e. a beamer
back-end that would want to keep latex snippets).  Hence, filtering
snippets is on back-ends side, and a new function is added to help
them in that task.