This requires mostly making moc a bit more permissive, which has the
advantage that it also simplifies the code a little bit.
The newly added test case demonstrates how to connect such a property
with a change signal.
One test case needed to be changed regarding the callback as the
publicProperty member now has a (permanent) observer and therefore
re-assigning the binding will re-evaluate it as the value might have
changed.
Change-Id: Ia7edcec432de830bdd4e07d943c5d4550c175ca4
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
