How to catch Block's onFocus event simply

I think it is more useful to implement focus event in Editor.js !
To catch Block's onFocus event, it needs to use MutationObserver currently.

```
render() {
const container = document.createElement('div');

setTimeout(() => {
const block = container.parentElement && container.parentElement.parentElement;

if (block) {
// TODO: disconnect
const observer = new MutationObserver(() => {
console.log(block.classList);
});

observer.observe(block, {
attributeFilter: ['class'],
attributes: true,
});
}
}, 100);

return container;
}
```

console output
```
DOMTokenList ["ce-block", value: "ce-block"]
DOMTokenList(2) ["ce-block", "ce-block--focused", value: "ce-block ce-block--focused"]
```