Introduce QFile(QUrl) constructor

People simply fail to get it right, I see it every month, either in #qt or in customer code. We need the API to make it easier and more foolproof. Otherwise users do QFile f(url.toString()), or other creative ways.

A QFile(QUrl) might not be the most elegant, since not all QUrls represent a local file, but neither is QFile(QString), as not all QStrings represent a path.

Not to mention qrc, where things get even more hairy, where the user must check for url.scheme() == "qrc" and prepend ":" himself, currently.