QQmlPropertyCache: adding setParent()

setParent() enables to dynamically set the parent of a
QQmlPropertyCache. This is used in the Qt Quick Designer to
create dynamic properties on an arbitrary object.

Change-Id: I0dafabcacf4222fc1bfe693736cbd23e1e928e8f
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Christiaan Janssen <christiaan.janssen@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>