Backwards compatibility limit

[olanti-p]

At the moment, we don't have a set limit on how backwards compatible the game saves and data scheme should be.

DDA tradition was to keep saves compatible between the stables, with no grace period for JSON data format changes.

Our approach so far has been a bit more relaxed: game saves should be compatible with DDA 0.E (I may be wrong on this, but I don't recall any breaking changes), while JSON format changes are is some cases provided with grace period (red debug message that yells "this will be changed", or "this has been changed"), but in other cases are immediate and hard break the external mods (usually the breaking PR provides instructions or scripts that should help with updating).

This could be subject to change in the future.