fix crash when removing Q_OBJECT header

We accidentally deleted artifact objects twice.

Change-Id: I8d7af93ca9733ada64fe6c882401eb0f80ace9d6
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
clear artifact timestamps after updating transformer

Otherwise those artifacts won't be rebuilt.

Change-Id: I005e2a8d489d45119a51e97d2cc5152bfef4bd41
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
fix checkForPropertyChanges for product values

checkForPropertyChanges always returned true, because we stored tracked
product properties with the module name "product".
Introduce the enum Property::Kind instead of abusing the module name.

Task-number: QBS-305
Change-Id: I51a1eb28f0bb3245d7e926685f3a0fe01f981959
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>