Enable browse-url to buttonize buffers

Enable browse-url to buttonize buffers

This patch also moves the internal/external browser logic from
shr to browse-url, where it belongs more naturally.

* doc/misc/eww.texi (Basics): Adjust to browse-url-external-browser.

* doc/misc/gnus.texi (Article Commands): Ditto.

* lisp/gnus/gnus-art.el (gnus-button-url-regexp): Default to
`browse-url-button-regexp', which has taken over the definition
previously enjoyed by this variable.

* lisp/gnus/gnus-sum.el (gnus-summary-browse-url): Adjust usage.

* lisp/net/browse-url.el (browse-url--browser-defcustom-type): New
internal variable...
(browse-url-browser-function, browse-url-external-browser): Used
by these two; the latter of which is a new variable.
(browse-url-botton-regexp): New variable.
(browse-url-button-map): New keymap.
(browse-url-button): New face.
(browse-url-add-buttons): New function to add clickable browse-url
buttons to almost any buffer.
(browse-url-button-open): New command.
(browse-url-button-copy): New command.

* lisp/net/eww.el (eww-browse-with-external-browser)
(eww-follow-link): Adjust usage.

* lisp/net/shr.el (shr-external-browser): Make an obsolete alias
of `browse-url-external-browser'.
(shr-browse-url): Adjust usage.