Store the is-a QObject fact with the metatype declaration.

This is a source incompatible change for Q_DECLARE_METATYPE(T*),
which now requires T to be fully defined.

The consequences of this are:
 * Forward declared types can no longer be declared as a metatype.
     (though this is a very uncommon thing to do).

There is a trivial workaround where necessary.

Change-Id: Id74c40088b8c0b466fcd7c55abd616f69acc82c8
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>