Add SVG icons for customize buffers

Add SVG icons for customize buffers

* etc/images/checkbox-mixed.svg:
* etc/images/checked.svg:
* etc/images/down.svg:
* etc/images/left.svg:
* etc/images/radio-checked.svg:
* etc/images/radio-mixed.svg:
* etc/images/radio.svg:
* etc/images/right.svg:
* etc/images/unchecked.svg:
* etc/images/up.svg: New files from the Adwaita Icon Theme (made by
the GNOME project).  The background color was changed from gray to
none to use the same colors as the current face instead.  (Bug#47074)
* etc/images/README: Add license information for the above new files.
* lisp/wid-edit.el (widget-image-conversion): Prefer SVG if it exists.
(radio-button): Rename radio buttons to "radio-checked" and
"radio".  These files did not exist before.