qmlplugindump: set isCreatable true in composite types

the qmlengine defines isCreatable as "is creatable through the C++
interface", for the qmltypes and qml code model it makes sense to
define isCreatable as "can appear as component in QML"

Change-Id: Ib26cf25dd5a70e1c03ce017e8ae572e8a9e562c1
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>