lisp/gnus/{eww,shr}.el: Merge changes made in Gnus master

lisp/gnus/{eww,shr}.el: Merge changes made in Gnus master

lisp/gnus/eww.el (eww-tag-select): Don't render totally empty <select> forms.
(eww-convert-widgets): Don't bug out if the first widget starts at the beginning of the buffer.
(eww-convert-widgets): Fix last patch.

lisp/gnus/shr.el (shr-insert-table): Respect border-collapse: collapse.
(shr-tag-base): Protect against base specs that are degenerate.
(shr-ensure-paragraph): Don't delete empty lines that have text properties, because these may be input fields.

lisp/gnus/eww.el (eww-convert-widgets): Put `help-echo' on input fields so that we can navigate to them.

lisp/gnus/shr.el (shr-colorize-region): Put the colours over the entire region.
(shr-inhibit-decoration): New variable.
(shr-add-font): Use it to inhibit text property decorations while doing preliminary table renderings.  This speeds up typical Wikipedia page renderings by 15%.
(shr-tag-span): Don't respect the <title>, because that overwrites the help-echo from links inside the spans.
(shr-next-link): Use `help-echo' for navigation, so that we can navigate to form elements, too.

lisp/gnus/eww.el (eww-button): New face.
(eww-convert-widgets): Use it to make submit buttons more button-like.