新增自定義連結類

RTE {
    classes {
        btn-lg {
            name = Button (large)
            requires = btn btn-default
        }

        btn-default {
            name = Button (default)
            requires = btn
        }
    }

    default {
        buttons.link.properties.class.allowedClasses := addToList(btn-lg, btn-default)
        proc.allowedClasses := addToList(btn-lg, btn-default)
    }
}