Refactor ThingsById system to use non static data

[Tick-git]

Label: 1.6, 1.5, Fix

The error in this PR probably didn’t occur often before, but with gravship it will happen more frequently since maps are generated and abandoned often. I also encountered this in #581, but couldn’t reproduce it at the time.

Issue:

The static data causes problems when two saves are played in one process.

public static Dictionary<int, Thing> thingsById = new();

In the first game, many maps are used, therefore some static cached things have a high mapIndex.

In the next game, fewer maps are used and one map is abandoned.

The static data from save before is not cleared correctly, and when map gets abandoned:

public class ThingsById

 public void UnregisterAllFrom(Map map)
 {
     thingsById.RemoveAll(kv => kv.Value.Map == map);
 }

calls a map getter that uses Find.Maps and tries to access it with the thing high mapIndex from the old save, which leads to an out-of-bounds error.

Fix

The PR removes the static data and introduces a non static ThingById class where the instance is part of the multiplayer game instance. This ensures we get a clean data structure with every new load. It also prevents direct access to the data structure by introducing wrapper methods for improved maintainability.