(widget-field-at): New function.

(widget-field-at): New function.
(widget-at, widget-field-activate): Use it.
(widget-tabable-at): Use `widget-at'.
(widget-specify-field): If the terminating character of the widget
  field (which is read-only) is a newline, put it into a special
  `boundary' field so that C-n/C-p act more naturally.
(widget-field-end): Also don't subtract one if a special
  `boundary' field has been added after the widget field.