Fix crash in QObjectWrapper

This can happen during QObject destruction, when
QObjectPrivate::deleteChildren() itself sets entries
in the children list to zero when deleting. These
zeros cause crash in markChildQObjectsRecursively().

Task-number: QTBUG-38635
Change-Id: I29ad9e793b78ca4e8d73fbb125f46db1b8292f20
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Add a QQuickWidget - QQuickView comparison example

Besides serving as a side-by-side test tool for QQuickView and
QQuickWidget (including multisampling), it also demonstrates some
useful practices for rendering 3D content via QQuickFramebufferObject.

Done-with: Paul Olav Tvete <paul.tvete@digia.com>
Change-Id: Ie73e998ee91e32ef1535dd6f0f65c8a69addcc5e
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>