Use QObject::d_ptr for public API classes

This follows the model used by the rest of Qt, potentially
avoiding binary compatibility issues.

The compromise is that we now depend on core-private, thus
forcing us to follow Qt's release cycle.

Change-Id: Ib2df51071fc35935ac99edf7b9c5562949cb43e2
Reviewed-by: Andras Becsi <andras.becsi@digia.com>