Interface improvements to cus-theme.el.

Interface improvements to cus-theme.el.

* cus-edit.el (custom-face-widget-to-spec)
(custom-face-get-current-spec, custom-face-state): New functions.
(custom-face-set, custom-face-mark-to-save)
(custom-face-value-create, custom-face-state-set): Use them.

* cus-theme.el (custom-theme--listed-faces): New var.
(customize-create-theme): Use *Custom Theme* as the buffer name.
Set revert-buffer-function.  Optional arg BUFFER.  Insert all
faces listed in custom-theme--listed-faces.
(custom-theme-revert): New function.
(custom-theme-add-variable, custom-theme-add-face): Insert at the
bottom of the list.
(custom-theme-write): Prompt for theme name if empty.
(custom-theme-write-variables): Use dolist.
(custom-theme-write-faces): Handle hidden (collapsed) widgets.