QQmlFileSelector doesn't have an extraSelectors method

The documentation states that if all you need to do is add extra selectors to the QQmlFileSelector, you don't need to create a separate QFileSelector instance to do it. That's nice, except if you want to query those extraSelectors - you can't without creating a QFileSelector for the QQmlFileSelector to act on and then query the QFileSelector instance what it's extraSelectors are.

Propose adding convenience functions to query the extraSelectors (and allSelectors) to QQmlFileSelector.