Improvements to face customization interface.

Improvements to face customization interface.

* lisp/cus-edit.el (custom-variable, custom-face): Doc fix.
(custom-face-edit): Add value-create attribute.
(custom-face-edit-value-create)
(custom-face-edit-value-visibility-action): New functions.  Hide
unused face attributes by default, and add a visibility toggle.
(custom-face-edit-deactivate): Show empty values with shadow face.
(custom-face-selected): Only use this for face specs with default
attributes.
(custom-face-value-create): Cleanup.

* lisp/wid-edit.el (widget-checklist-value-create): Use dolist.
(widget-checklist-match-find): Make second arg optional.