fix calculation of added/removed artifacts

We kept lists of added and removed artifacts in ProductBuildData. It was
never quite clear when to invalidate those lists, which led to QBS-635.
Instead we let the RuleNode decide which artifacts it considers as
"added or removed for this rule".

Task-number: QBS-635
Change-Id: I390e0ab775c695045c6e91ade3ac7326692cb314
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>