Re: [w3c/webcomponents] connectedCallback timing when the document parser creates custom elements (#551)

@franktopel it's useful for empty nodes with shadow dom, and not much else indeed.

With `parsedCallback` you can setup sure that attributes are there as well.

Meanwhile, if you react to `attributeChangedCallback`, if `this.parsed` is false you should queue the operations if important for the component state.

```js
const attributeChanged = new WeakMap;
class MyEl extends HTMLParsedElement {
  parsedCallback() {
    // setup the node
    const changes = attributeChanged.get(this);
    if (changes) {
      attributeChanged.delete(this);
      changes.forEach(args => this. attributeChangedCallback.apply(this, args));
    }
  }
  attributeChangedCallback(name, oldVal, newVal) {
    if (!this.parsed) {
      const changes = attributeChanged.get(this) || [];
      changes.push([name, oldVal, newVal]);
      if (changes.length === 1)
        attributeChanged.set(this, changes);
      return;
    }
    // the rest of the code
  }

  connectedCallback() {
    // here you can safely add listeners
    // or set own component properties
    this.live = true;
  }

  disconnectedCallback() {
    // here you can safely remove listeners
    this.live = false;
  }
}
```



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