Fix regression with nested objects served by custom parsers

Fix regression with nested objects served by custom parsers

The following piece of code used to work and broke with Qt 5.3:

ListModel {
    property var conn: Connection {
        ...
    }
}

When validating the properties of the ListModel we would not validate
the Connection sub-object here, which meant the custom parser for the
connection object was never called.

We need to extend the logic for sub-object validation to recursive into
sub-objects when this is either an attached property (Component.onComplete on a
list model for example) or the object is assigned to an _existing_ property,
i.e. a property not deal with by the custom parser. In this case that's a
custom declared property.

Change-Id: Ic99f746f08771460cc6424a9e8a839c78a7eafd9
Task-number: QTBUG-45735
Reviewed-by: Lars Knoll <lars.knoll@digia.com>