Support extended menus in widget-choose

Support extended menus in widget-choose

* doc/misc/widget.texi (Utilities): Document widget-choose.

* etc/NEWS: Document the feature (bug#4787).
* lisp/wid-edit.el (widget--simplify-menu): New function, to convert
extended menus into simplified menus when using the menu to prompt
through the minibuffer.
(widget-choose): Accept a keymap menu.  When not using x-popup-menu,
simplify the menu with widget--simplify-menu.  Document the changes in
the docstring.