Clear the chain of incubated objects in QQmlIncubatorPrivate::clear

Change-Id: I432310c0e6006d567fd59b4b1021a9e1538ef78f
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Compilation fix

gcc 4.6 chokes on this construct.
Replace with a shorter and more readable one :)

Change-Id: I02ff7a2c5300de656e61ee5ca8effabdd74991bb
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>