Rewrite Save Data Management

[doczi-dominik]

Relying on the intricacies of the Haxe Serializer turned out to be insufficient for this use-case. To ensure the most compatibility between updates, GelaVolt should use a simple, custom serializing format that only stores data that is different from the default, i.e. overrides.

Another nice addition would be a save data versioning system and a migration pipeline. If the current save is v5 and GelaVolt loads a v2, it should be able to migrate v2 -> v3 -> v4 -> v5.