C++: generate quick-exit style setters for Q_PROPERTY.

Change-Id: Ie278c2901a49075500526eb17a84ab2baf80cd01
Task-number: QTCREATORBUG-12519
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>