Allow hiding of modified custom widgets.

Allow hiding of modified custom widgets.

* cus-edit.el (custom-variable, custom-face): Combine the
:inhibit-magic and :display-style properties into a single
:custom-style property.
(custom-toggle-hide-variable, custom-toggle-hide-face): New
functions.  If hiding an edited value, save it to :shown-value.
(custom-variable-value-create, custom-face-value-create): Use
them.
(custom-magic-reset): Allow magic property to be unset.

* cus-theme.el (custom-theme-add-var-1)
(custom-theme-add-face-1): Use the :custom-style property.

* custom.el: (custom-theme-load-path): Doc fix.