Expose information about target artifacts in the public API.

In turn, information about file tags and properties is being removed
from the ProductData class, because it can now be gathered from the
TargetArtifact objects.

Change-Id: Iea66dd19f665ada97879c26c9916deb072db8ba8
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>