Generate proper property constants for legacy property IDs

[le-cds]

As a property's ID changes it can be annotated with its legacy ID. If this property is referred to by its legacy ID, as it may be in graphs serialized before the ID change, this works well since ELK still knows about the old ID. It breaks binary code compatibility, though: code that accessed the old generated property constant won't compile any more since the constant's name will have changed along with the ID.

We could remain backwards compatible by generating property constants for legacy IDs as well. Those would be marked as deprecated and could be removed after one major release, but would give users of our library a grace period before their code stops to compile.

I guess the question of whether or not to invest the extra work is a bit philosophical: do we want to give people a warning before things break, or do we want things to fail early?