tree.List: smarter check if draggable & sortable are set to true

Since we can dynamically both configs at any point, a ctor based check like this is not sufficient:

/**
 *
 * @param config
 */
constructor(config) {
    super(config);

    let me = this;

    if (me.draggable && me.sortable) {
        console.error('tree.List can be either draggable or sortable, not both.', me.id);
    }
}

I will move the check into afterSetDraggable() & afterSetSortable() instead.