To hide the IsPointerToTypeDerivedFromQObject monstruosity :-)

Documentation for Q_DECLARE_METATYPE and qRegisterMetaType was updated
to mention requirements on registered types and how they can be
circumvented for pointer types with the new macro.

Change-Id: If83b037a8e2f28761eb903525e87008107298801
Reviewed-by: Harald Fernengel <harald.fernengel@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>