Special handling of the `custom-enabled-themes' custom variable.

Special handling of the `custom-enabled-themes' custom variable.

* lisp/custom.el (custom--inhibit-theme-enable): Make it affect only
custom-theme-set-variables and custom-theme-set-faces.
(provide-theme): Ignore custom--inhibit-theme-enable.
(load-theme): Enable the theme explicitly if NO-ENABLE is non-nil.
(custom-enabling-themes): Delete variable.
(enable-theme): Accept only loaded themes as arguments.  Ignore
the special custom-enabled-themes variable.
(custom-enabled-themes): Forbid themes from setting this.
Eliminate use of custom-enabling-themes.
(custom-push-theme): Quote "changed" custom var entry.