QFixed should be a structural type

I'm classifying this as a bug, because there's no reason it shouldn't be.

Background:
C++20 allows class types as non-type template arguments and I found it useful to use a QFixed there, but that fails because QFixed isn't "structural". It needs a defaulted operator==, and members that are public.

I'm not sure this is fixable in Qt 6, but it should be fixed for Qt 7, at least (but see epic for why we may want to wait).