org-element: Improve timestamps parsing

org-element: Improve timestamps parsing

* lisp/org-element.el (org-element-timestamp-parser): Modify timestamp
  objects properties.
(org-element-headline-parser, org-element-inlinetask-parser): Remove
`:timestamp' and `:clock' property.  Add `:clockedp' property.  Also,
set `:closed', `:deadline' and `:scheduled' values to timestamp
objects, not strings.  Small refactoring.
(org-element-clock-parser): Rename `:time' property into `:duration'.
Also, set `:value' value as a timestamp object, not a string.
(org-element-planning-parser): Set `:closed', `:deadline' and
`:scheduled' values to timestamp objects, not strings.
(org-element-clock-interpreter, org-element-planning-interpreter)
(org-element-timestamp-interpreter): Update interpreters.
(org-element--current-element): Tiny refactoring.
* testing/lisp/test-org-element.el: Add tests.