org-element: Store fixed-with and comment contents without markers
* contrib/lisp/org-element.el (org-element-comment-parser): Store
comments without leading hash and maybe plus sign.
(org-element-fixed-width-parser): Store fixed-width contents without
leading colons.
(org-element-comment-interpreter,
org-element-fixed-width-interpreter): Apply changes.
* contrib/lisp/org-e-odt.el (org-e-odt-fixed-width): Apply changes to
fixed-width `:value' property.
* contrib/lisp/org-e-latex.el (org-e-latex-fixed-width): Apply changes
to fixed-width `:value' property.
* contrib/lisp/org-e-html.el (org-e-html-fixed-width): Apply changes
to fixed-width `:value' property.
* contrib/lisp/org-e-ascii.el (org-e-ascii-fixed-width): Apply changes
to fixed-width `:value' property.
* testing/lisp/test-org-element.el: Add tests.