Mod checksums

**What do you want to see?**
Add a checksum to mods that is saved in the game state file. If a player attempts to load a game and their copy of the mod has the wrong checksum, warn them (don't crash like in SE4) that their mod files may be out of date. This checksum should be generated from all the data files in the mod; graphics and music can be left out.

**How would it help players?**
Keep players informed when their mod is out of date.

**Alternatives**
Crash when the checksum doesn't match, like SE4? LOL I dunno...

**Screenshots and mockups**
N/A

**Things to consider**
N/A