More face customization cleanups.

More face customization cleanups.

* cus-edit.el (custom-commands, custom-buffer-create-internal)
(custom-magic-value-create): Pad button tags with spaces.
(custom-face-edit): New variable.
(custom-face-value-create): Determine whether to use the usual
face editor here, instead of using custom-face-selected.  Pass
face defaults to custom-face-edit widget.
(custom-face-selected, custom-display-unselected): Delete widgets.
(custom-display-unselected-match): Function removed.
(custom-face-set, custom-face-mark-to-save): Accept
custom-face-edit widgets as the direct widget child.

* wid-edit.el (widget--completing-widget): New var.
(widget-default-complete): Bind it when doing completion.
(widget-string-complete, widget-file-complete): Use it.