Consistent support for random seeds.
Currently, setting of random seeds is done in different places, and possibly with different logics inspired by different philosophies.
By default, simulations should be reproducible, i.e. always have exactly the same result. User-provided or non-deterministic seeds should only be used when explicitly required (through new API and GUI functionality).
Possibly, we should use one rng per thread. How this is to be done is an open DESIGN QUESTION.
Precondition: #1141 should be done before.