avoid warnings regarding resource_ids when rereading event resources

This PR gets rid of the numerous warnings when an event resource gets read more than once and the respective `ResourceIdentifier`s are in the weak ref dictionary already. One possible remaining problem is that more than one referred object exist in that case and if the equality check `==` is not sufficient to really ensure equality, then accessing the referred object might not yield the expected result (but that problem exists in master too). The same holds for multiple objects with same resource id text that start out the same and get changed afterwards. I even tend to think we should disallow making changes to event type objects that have a fixed resource id..