Bring back QXmlSchema and QXmlSchemaValidator or similar functionalities

A well-formatted XML file does not necessarily follow a valid schema. In Qt5 we had QXmlSchema to validate that. But we removed that from Qt6 and had never provided a replacement. But in practice, validating schema is a common requirement.

Some third-party libraries/tools that may be related: libxml2, XSD/e. Or maybe just revise what we did in Qt5 and put it in a more "Qt6" way.

Extra point: fast exit as soon as validator finds a violation and skips the rest of the file.