Updating old Save Files

[Katniss218]

Problem:
Save files will become incompatible if the mod used updates and changes the save file format or structure.

Solution:
Introduce a 'Data Fixer' in every mod, that takes in a (old_version, new_version) pair, and transforms the given save file accordingly.

The old version will be taken from the save itself, and the new version from the version that's currently loaded.

The DataFixer may only support certain pairs of versions, which needs to be communicated out, to display to the player.