For discussion: more powerful IDummyDefinition

As discussed in #402.

This isn't the proposed final implementation, just something to see whether @FakeItEasy/owners like or hate the idea.
because 3 is bigger than 0. Note that this was only necessary because `DomainEventDummyDefinition` and `DomainEventDummyDefinition` would fight over `RobotRunsAmokEvent`s. Existing classes that implement `DummyDefinition<T>` would not have to start supplying a custom `Priority` value.