(mu-cite-ml-count-field-list): Modify tag string for `choice';

(mu-cite-ml-count-field-list): Modify tag string for `choice';
use `const' with the arg nil for the menu separator;
delete element which value is nil when the symbol value is set.
(mu-cite-top-format): New implementation for customization using
`mu-cite-choose-top-format' and `mu-cite-custom-set-variable'.
(mu-cite-prefix-format): New implementation for customization using
`mu-cite-choose-prefix-format' and `mu-cite-custom-set-variable'.
(mu-cite-custom-set-variable): New function.
(mu-cite-choose-top-format): New widget.
(mu-cite-choose-prefix-format): New widget.

(widget-mu-cite-method-prompt-value): Abolish function.
(mu-cite-method): Abolish widget.
(widget-mu-cite-method-prompt-value-history): Abolish variable.
(mu-cite-method-list): Abolish function.