Function: define-widget

Define a new widget type named NAME from CLASS.

NAME and CLASS should both be symbols, CLASS should be one of the
existing widget types, or nil to create the widget from scratch.

After the new widget has been defined, the following two calls will
create identical widgets:

* (widget-create NAME)

* (apply 'widget-create CLASS ARGS)

The third argument DOC is a documentation string for the widget.