org-element: Recognize planning info only right after a headline
* lisp/org.el (org-planning-line-re): New variable. Renamed from
`org-planning-or-clock-line-re'.
* lisp/org-element.el (org-element-paragraph-separate,
org-element-headline-parser, org-element-inlinetask-parser, ,
org-element--current-element): Use new variable.
(org-element-planning-parser): Check previous line before parsing
planning info. When right after a headline, fallback to paragraph
parsing.
* testing/lisp/test-org-element.el (test-org-element/planning-parser):
Add a test. Update others.
(test-org-element/timestamp): Remove a test.
* testing/lisp/test-ox.el (test-org-export/handle-options): Update
test.