layout.Flexbox: only apply configs to wrapperCls in case there are changes

this will reduce the afterSetWrapperCls() calls by a lot.

when creating a new layout, container.Base will trigger:

    afterSetLayout(value, oldValue) {
        let me = this;

        if (me.rendered) {
            oldValue.removeRenderAttributes();
            value.applyRenderAttributes();

            me.items.forEach((item, index) => {
                oldValue.removeChildAttributes(item, index);
                value.applyChildAttributes(item, index);
            });
        }
    }

=> the initial values will get applied anyway