Implement save file guarding

[conorlawton]

Add meta data to saves to prevent them being loaded without the mods present.

• When the user saves their game, include a list of currently installed mods.
• When the user loads that save, check all the mods are currently loaded.
• If a mod on the save's list of mods is not loaded, warn the user that the mod isn't present, but give them the option to force load it anyway.
• Add a new field to the mod manifest to allow mod authors to mark their mods as not altering save data. This should be opt-out.

[2767mr]

Most mods do not actually affect the save file at all

[conorlawton]

This is for mods that do. Examples are any mods where the player saves in a modded location or saves while using a non-vanilla character model. This would cause the game to error at runtime if those mods aren't present.

[elluminance]

I'm pretty sure their point was to have this feature be opt-in rather than opt-out - not to not have this feature at all.

[2767mr]

It is something that is related to this feature

[conorlawton]

Opt-out covers more ground than opt-in. It is better for the end user to let the modloader assume that any given mod could alter their save, as this will mitigate the runtime errors resulting from malformed save data.