As a user of qdoc, I want to be able to \relates _classes_ with other classes.

ATM, I can't (cf. doc check failure on https://codereview.qt-project.org/c/qt/qtbase/+/458459/4).

This will be needed once we start providing std::hash specializations for Qt types (QTBUG-33428), and probably other things.

We don't need to place the class docs on the \relates'ed class' type, but it would be nice if such a \class would inherit any meta data from the "parent" class and be shown (as a class) in the "Related Non-Members" or a new "Related Classes" section.