* lisp/ox.el (org-export-as): Call `inner-template' function, if
available.
* lisp/ox-html.el (org-html-inner-template): New function.
(org-html-template): Move all parts that should be inserted even in
a body-only export into `org-html-inner-template'.
* testing/lisp/test-ox.el: Add tests.
* lisp/org.el (org-modules): Remove export back-ends from the list.
Update docstring.
(org-export-backends): New variable.
* lisp/ox.el (org-export-async-start): Make sure export framework is
required in the external process.
* lisp/ob-exp.el (org-babel-exp-process-buffer): Renamed from
`org-export-blocks-preprocess'.
* lisp/ox.el (org-export-execute-babel-code): Apply previous renaming.
* testing/org-test.el (org-test-at-id): Make sure the function returns
the value of the last form in its body.
* testing/lisp/test-ob-exp.el: Fix tests.
* testing/lisp/test-ob-lob.el: Fix tests.