Children attribute mutations

Most of composite elements would want to know their children, which can be easily monitored using mutation observer:

```
mutationObserver.observe(this.element, {childList: true});
```

This works well when the component does not differentiate the types of children. E.g. "all children are carousel's slides". When children have different designations we can use attributes to assign them different roles. For instance, a customized "next" button in the carousel can be marked up as ``. If these attributes can change after the initialization, we may need to observe each child for attribute changes as well. E.g.

```
mutationObserver.observe(child, {attributes: true, attributesFilter: ...});
```

The goal is to avoid this as much as possible given the additional performance and code management cost.

この issue ではファイル、テスト、エントリーポイントが指定されていません。レポートにある composite-element と MutationObserver の例から始めてください。完了とするには、不要な監視を行わずに子要素の属性変更を処理するための、決定済みで実行可能なスコープが必要です。

索引モデルが issue の本文から書いたものです。