(custom-push-theme): Maintain list of the settings

(custom-push-theme): Maintain list of the settings
of a given theme in its theme-settings property.
Maintain position of old settings in the theme-value or theme-face property.
(custom-enabled-themes): New variable.
(custom-theme-enabled-p): New function.
(provide-theme): Update custom-enabled-themes.
Disable and reenable the `user' theme.
(require-theme): Doc fix.
(custom-do-theme-reset, custom-remove-theme): Functions deleted.
(custom-theme-value, custom-theme-variable-value): Likewise.
(custom-theme-reset-internal): Likewise.
(custom-theme-load-themes): Fix bugs and use custom-disable-theme.
(custom-enable-theme, custom-disable-theme): New functions.
(custom-variable-theme-value, custom-face-theme-value): Likewise.
(custom-theme-recalc-variable, custom-theme-recalc-face): Likewise.
(custom-theme-reset-variables): Simplify.
(deftheme, custom-declare-theme, custom-make-theme-feature):
Definitions moved.