org-element: Use strict comment syntax (no "#+" allowed)

org-element: Use strict comment syntax (no "#+" allowed)

* lisp/org.el (org-fill-paragraph): No need to use
  `org-element-paragraph-separate' in a verse block since blank lines
  only can end a "paragraph".
* lisp/org-element.el (org-element-paragraph-separate): Apply changes
  to comments.
(org-element-paragraph-parser): Correctly find end of paragraphs.
(org-element--current-element): Require colons for Babel calls.
(org-element-center-block-parser, org-element-dynamic-block-parser,
org-element-quote-block-parser, org-element-special-block-parser,
org-element-comment-block-parser, org-element-example-block-parser,
org-element-export-block-parser, org-element-src-block-parser,
org-element-verse-block-parser): Fall-back to paragraph parsing when
incomplete or ill-formed.
* testing/lisp/test-org-element.el: Update tests.