Convert shr.el from using overlays into using text properties

Convert shr.el from using overlays into using text properties

* eww.el (eww-mode-map): Use `shr-next-link' (etc) instead of the
widget commands, since we're no longer using widgets for links.

* mm-decode.el (mm-convert-shr-links): New function to convert
new-style shr URL links into widgets.
(mm-shr): Use it.

* shr.el (shr-next-link): New command.
(shr-previous-link): New command.
(shr-urlify): Don't use `widget-convert', because that's slow.
(shr-put-color-1): Use `add-face-text-property' instead of overlays,
because collecting the overlays and reapplying them when generating
tables is slow.
(shr-insert-table): Ditto.