* lisp/gnus/gnus-sum.el (gnus-summary-widget-forward)
(gnus-summary-button-forward, gnus-summary-widget-backward)
(gnus-summary-button-backward, gnus-collect-urls-primary-text)
(gnus-collect-urls, gnus-summary-browse-url): Stop using widgets
and star using button.el buttons instead.
* lisp/gnus/mm-decode.el (mm-shr, mm-handle-filename): Don't
convert shr buttons into widgets.