org-element: Add `clock' and `planning' element types

org-element: Add `clock' and `planning' element types

* contrib/lisp/org-element.el (org-element-babel-call-parser): Fix
  property name.
(org-element-babel-call-interpreter, org-element--element-block-re):
Fix docstring.
(org-element-clock-parser, org-element-clock-interpreter,
org-element-planning-parser, org-element-planning-interpreter): New
functions.
(org-element-time-stamp-parser): Move planning keywords out of the
object: no more `:appt-type' property
(org-element-time-stamp-interpreter,
org-element-time-stamp-successor): Apply changes to previous function.
(org-element-paragraph-separate): Time keywords also end paragraphs.
(org-element-all-elements): Register new elements types.
(org-element-current-element): Recognize new elements.
(org-element-parse-elements): Fix comments.
* testing/lisp/test-org-element.el: Add tests.