(mu-cite-ml-count-field-list): Don't use `mu-cite-custom-set-variable'.

(mu-cite-ml-count-field-list): Don't use `mu-cite-custom-set-variable'.
(mu-cite-top-format): New implementation for customization.
(mu-cite-prefix-format): Ditto.
(mu-cite-custom-set-variable): Remove function.
(mu-cite-choose-top-format): Remove widget.
(mu-cite-choose-prefix-format): Remove widget.
(TopLevel): Don't use `mapcar' for defining obsolete functions.
(TopLevel): Undo the last change for defining obsolete variables under XEmacs.
(mu-cite-obsolete-variable-alist): Redefine as a macro which will be bound at
the compile time only.