Add version to game objects

[jabelch]

Each time we make a change to the game (adding objects, changing how we handle randomness) the version will change. This can be an issue for saved games and the state.
We need a method to basically "look" at the version of a game and revert to that state. One option would be to use a const.py factory.

[BlackVegetable]

In some cases, a client with an outdated version could function properly with a newer server, but I don't think we should count on that happening. What is a const.py factory?

[BlackVegetable]

This only needs to apply to save games, right?

[malikoth]

Realistically, it should also apply to the network protocol. The solution as I've seen it in other games is to never connect a client and server if they are running two different versions, and show a message on the side with the older release that it's time to update. For saves, create a method specific to each change you implement that manually adjusts the data structure to convert from the old version to the new.