More cleanups and minor fixes for Customize.

More cleanups and minor fixes for Customize.

* cus-edit.el (custom-face-edit-fix-value): Use
custom-fix-face-spec.

* custom.el (custom-push-theme): Cleanup (use cond).
(disable-theme): Recompute the saved-face property.
(custom-theme-recalc-face): Follow face alias before setting prop.

* custom.el (custom-fix-face-spec): New function; code moved from
custom-face-edit-fix-value.
(custom-push-theme): Use it when checking if a face has been
changed outside customize.
(custom-available-themes): New function.
(load-theme): Use it.

* image.el (image-checkbox-checked, image-checkbox-unchecked): New
variables, containing checkbox images.

* startup.el (fancy-startup-tail):
* wid-edit.el (checkbox): Use them.