Decrease compile time impact of qtypeinfo.h

qtypeinfo.h currently includes heavy headers like variant, optional and tuple. However, those are only needed for the traits in namespace QTypeTraits, which are rarely used.

For QTypeInfo, we only really need type_traits, though. Moreover, Q_DECLARED_SHARED needs even less: only a forward declared QTypeInfo class, and the enum with the typeinfo flags.

It therefore makes sense to split the header/move parts out of it.