Extend the ReturnedValue mechanism to pointers to Managed objects

Extend the ReturnedValue mechanism to pointers to Managed objects

Add a Returned<T> that we can return instead of raw pointers
to Managed objects.

Start using the Returned<T> for a few methods.

Also clean up all our classes to use the Q_MANAGED macro instead
of manually defining their vtable.

Change-Id: I0a2962e47f3de955cd2cd8474f8f3fcc9e36d084
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>