Stop using Qt private API

[special]

The only use of private Qt API is QMetaObjectBuilder. If that is fixed, we can get proper ABI compatibility and not risk breakage as Qt moves forward.

The output of QMetaObjectBuilder is the moc-style metaobject structures. They aren't trivial, but the format is covered under Qt's ABI. If the code is extracted, it can be used safely.

There are some complications in pulling that out of Qt where it's using other private API, but none that seem insurmountable.