Backport of:

commit 354d3a17e2a772c467343a47f7b328ea8482f7fa
Author: Albert Astals Cid <albert.astals@canonical.com>
Date:   Thu Mar 13 17:09:39 2014 +0100

Clear the chain of incubated objects in QQmlIncubatorPrivate::clear

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