Function setting_fn 

pub fn setting_fn(
    child: impl IntoUiNode,
    wgt_fn: impl IntoVar<WidgetFn<SettingArgs>>,
) -> UiNode

P Widget function that converts SettingArgs to a setting editor entry on a settings list.

Note that the widget must set setting or some features will not work.

Sets the SETTING_FN_VAR.