org-element: List interpretation doesn't depend on its structure

org-element: List interpretation doesn't depend on its structure

* lisp/org-element.el (org-element-item-interpreter): Simplify bullet
  creation.
(org-element-plain-list-interpreter): Fix wrong bullets, if needed.

This change allows to intrepret plain lists without providing list's
structure, which can be a bit hairy.  For example, the following
snippet now suffices to create a list programmatically:

  (org-element-interpret-data
   '(plain-list nil
                (item (:bullet "-") (paragraph nil "a"))
                (item (:bullet "-") (paragraph nil "b"))))