(thing-at-point): Use `thing-at-point' property, if any.

(thing-at-point): Use `thing-at-point' property, if any.
(bounds-of-thing-at-point): Use `bounds-of-thing-at-point' property.
(thing-at-point-bounds-of-url-at-point): New function.
(thing-at-point-looking-at): New function, adapted from old
browse-url-looking-at.
(thing-at-point-url-at-point): New function, adapted from
browse-url-url-at-point.
(thing-at-point-url-chars): Variable deleted.
(thing-at-point-url-path-regexp, thing-at-point-short-url-regexp,
thing-at-point-url-regexp, thing-at-point-markedup-url-regexp):
New variables.
(url): `beginning-op' property function changed to use
`thing-at-point-bounds-of-url-at-point'.  `end-op' property no
longer set -- functionality no longer supported for the more
sophisticated treatment of URLs so `forward-thing' no longer works
in this case.