Generators should have a version number

[LogicalError]

Since generators create all their brushes from scratch, any change in behaviour could potentially drastically change how a level created using generators would look. So we cannot simply change a generator once people have already started using it, but we might end up in situations where we feel we really do need to improve a generator. By adding version numbers we can make it possible to handle scenarios like upgrading/converting generators, or having different behaviours for different versions of generators. This gives us more control in situations like this.