Add new commands to describe buttons and widgets

Add new commands to describe buttons and widgets

* lisp/help-fns.el (describe-widget-functions): New variable, used by
describe-widget.
(describe-widget): New command, to display information about a widget.
* lisp/button.el (button-describe): New command, for describing a button.
(button--describe): Helper function for button-describe.
* lisp/wid-edit.el (widget-describe): New command, for describing a
widget.
(widget--resolve-parent-action): Helper function, to allow
widget-describe to display more useful information (bug#139).