Fix State button for customize-icon (Bug#66635)

Fix State button for customize-icon (Bug#66635)

* lisp/cus-edit.el (custom-icon-action): New function.
(custom-icon): Use it as the :action.  Otherwise, clicking the State
button is a noop.  Remove irrelevant stuff from the docstring and
comment out some copy-pasta.
(custom-icon-extended-menu): New variable, the menu to show upon
:action.
(custom-icon-set): Really redraw the widget with the new settings.
Comment out strange call to custom-variable-backup-value.
(custom-icon-save): New function.

* lisp/emacs-lisp/icons.el (icons--merge-spec): Fix call to plist-get
and avoid infloop.