Make QSignalTransition take a pointer-to-const QObject

Make QSignalTransition take a pointer-to-const QObject

The obvious idea is that a connect() happens behind the scenes.
As QObject::connect takes a pointer-to-const, QSignalTransition should
do that as well.

TODO: the API becomes asymmetric in that it takes
a "const QObject *" but returns a "QObject *". Reasoning is needed.

Change-Id: I18d0436e7036eee851fd36d5b8ccda4a4757938f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>