Introduce AreasRecycleBin instead of deleting unloaded AreaStats

[Wartori54]

Fixes #989. Introduces a new field in LevelSetStats which stores the AreaStats that have no matching loaded AreaData. This makes it so the extra AreaStats are not deleted but instead moved elsewhere, then each time save data is loaded those AreaStats are taken into account again and potentially moved back to the main Areas list.

Since this PR touches SaveData code, I'd appreciate if it could get three approvals instead before merging.

[DashingCat]

I was able to reproduce the linked issue (#989) and in my testing this PR fixes the issue.

I would maybe add a few things to make the code a bit clearer :

• some logging when adding an area to areasBin;
• some kind of constants for int.MaxValue and int.MaxValue - 1, maybe like missingAreaID and subAreaID;
• a comment explaining how an area can end up in the areasBin.

[Wartori54]

@DashingCat thank you for your review, I have applied the feedback from your comment in the latest commit. I have also added xmldocs and logging for the very similar LevelSetRecycleBin, since otherwise it felt incorrect to add logging for areas but not levelsets.