Avoid unnecessary nesting in ol.Overlay

So that the positioned element is the one passed to `ol.Overlay`, not its parent.

One of my motivation to do it is to be able to set CSS transitions on my overlay’s element:
```css
.overlay {
  transition: top .2s, left .2s;
}
```

And then to have animated transition when setting position.
I’ve moved it in `insertElement_`.