Moved `Controller::prepareForWidget()` to `Widget::setAttributesFromDca()`.

Make sure to always call the inherited method of the specific widget class and
not the one of the parent class! Thanks to PHP's late static binding, this will
allow widget classes to override the routine:

```
// Wrong
new TextField(Widget::setAttributesFromDca(…));

// Correct
new TextField(TextField::setAttributesFromDca(…));
```

See #4697 for more information.