QDomDocument: treat as whitespaces only what's defined in the XML spec

When parsing an XML document, QDomDocument treats as whitespaces all characters for which QChar::isSpace() is true, which isn't complaint with the spec. Only the characters listed in https://www.w3.org/TR/xml/#NT-S should be recognized as whitespaces. But changing this in Qt 6 might break a lot of code that was relying on the wrong behavior. To avoid this, we can introduce a new ParseOption for QDomDocument, so that the users can choose to make the behavior compliant with the XML spec, if needed. In Qt 7, we can make this a default behavior.