Bugfixes for `customize-create-theme'.

Bugfixes for `customize-create-theme'.

* cus-theme.el (customize-create-theme): Delete overlays after
erasing.  If given a THEME arg, display only the faces of that arg
instead of custom-theme--listed-faces.
(custom-theme-variable-menu, custom-theme-variable-action)
(custom-variable-reset-theme, custom-theme-delete-variable): Deleted.
(custom-theme-add-variable, custom-theme-add-face): Apply value
from the theme settings, instead of the current value.
(custom-theme-add-var-1, custom-theme-add-face-1): New functions.
(custom-theme-visit-theme): Allow calling outside theme buffers.
(custom-theme-merge-theme): Don't enable the theme when merging.
(custom-theme-write-variables, custom-theme-write-faces): Use the
:shown-value properties to save buffer values, not global ones.
(customize-themes): Display a warning about user customizations.

* cus-edit.el (custom-variable-value-create)
(custom-face-value-create): Obey new special properties
:shown-value and :inhibit-magic.