Add missing metatype registration.

The code path where this is relevant has apparently never been taken, so
add an autotest as well (and fix the wording of the error message).

Change-Id: I314f1cb888264d30920a58f4e9cbbbf01fbcc4e6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Take product type changes into account when change tracking.

This property is special: We cannot just test for relevance by
checking whether it is used in any transformers, because changing
it can cause new transformers to get pulled in.

Task-number: QBS-650
Change-Id: If18495b47c7238fc8a1256146f4e935ce9655b18
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>