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.

El issue no menciona archivos, pruebas ni puntos de entrada. Empieza por los ejemplos de composite-element y MutationObserver del informe; para darlo por terminado se requiere un alcance decidido y accionable para gestionar los cambios en los atributos de los elementos hijos sin observación innecesaria.

Escrito por el modelo de indexación a partir del texto del issue.