Feature: #51905 - Add dependencies between classes in the Rich Text Editor

RTE.classes.[ *classname* ] {
    .requires = list of class names; list of classes that are required by the class;
        if this property, in combination with others, produces a circular relationship, it is ignored;
        when a class is added on an element, the classes it requires are also added, possibly recursively;
        when a class is removed from an element, any non-selectable class that is not required by any of the classes
        remaining on the element is also removed.
}