org.el: improve org-property-re and use it throughout
* lisp/org.el (org-property-re): Improve definition so that this regex
can be used in all situations. Extend docstring with explanation of
matching groups.
(org-at-property-p): Implement using `org-element-at-point'.
(org-entry-properties, org-buffer-property-keys, org-indent-line):
Use `org-property-re' and adjust match group numbers accordingly.
* lisp/org-element.el (org-element-node-property-parser): Use
`org-property-re' and adjust match group numbers accordingly. Move
`looking-at' out of the let clause to not rely on the unspecified
evaluation order inside the let.