Fix QQmlProperty to work correctly with deeply nested objects.

Don't keep reusing the top-level object to look up nested properties.
This allows code such as the following to work correctly:

PropertyChanges {
    target: myObject
    textChild.font.pixelSize: 24
}

Change-Id: I39e52dbc20d2409fc756a36e668fcf664eb1905f
Reviewed-by: Brett Stottlemyer <bstottle@ford.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>