Implement Modal Remove Method

[Deanosaurus]

The inherited remove method does not target the body but the widget itself. It would be nice to have a remove similar to that of the add that detects instance of ModalFooter and otherwise directs the remove to the child widgets of the body.

[Deanosaurus]

Perhaps

@Override
    public void remove(Widget w) {
        if (w instanceof ModalFooter) {
            super.remove(w);
        } else {
            body.remove(w);
        }
}

[reinert]

Could you state some use case in order to justify this need?

Notice that this change would cause some confusion with other methods dealing with inside widgets. How should getWidgetCount behave?

I do think we need to improve the treatment of ModalFooter, specially because it is used only inside Modal. Maybe it never should be public instantiated. We could find another way of better deal this widget with Modal.