Port QMimeData data()/setData()/hasFormat() to QAnyStringView format

QMimeData doesn't (necessarily) store the QString format that its data() and setData() take, so it's a perfect candidate for porting to QAnyStringView.

Alas, it has a protected virtual retrieveData() that makes this change hard to do in Qt 6 (would have to use slots in order to emulate virtual functions, not sure this would pull it's weight). In Qt 7, we can just change the argument.