Support text overlays for thingatpt provider helpers

Support text overlays for thingatpt provider helpers

* lisp/thingatpt.el (thing-at-point-for-text-property)
(forward-thing-for-text-property)
(bounds-of-thing-at-point-for-text-property): Rename to...
(thing-at-point-for-char-property)
(forward-thing-for-char-property)
(bounds-of-thing-at-point-for-char-property): ... and add overlay
support.  Update callers.

* test/lisp/thingatpt-tests.el (thing-at-point-providers)
(forward-thing-providers, bounds-of-thing-at-point-providers): Test
overlays too.

* test/lisp/progmodes/bug-reference-tests.el (test-thing-at-point): Test
'bounds-of-thing-at-point' and 'forward-point'.

* etc/NEWS: Update function names in announcement.