manager.Component: getChildren() => smarter logic required

the original implementation is really old to be fair:

getChildren(component) {
    let childComponents = [],
        childNodes      = VNodeUtil.getChildIds(component.vnode),
        childComponent;

    childNodes.forEach(node => {
        childComponent = this.get(node);

        if (childComponent) {
            childComponents.push(childComponent);
        }
    });

    return childComponents;
}

it is parsing the vnode, which does not exist before a component has been rendered. instead, we want to search the collection recursively for parentId matches.