Let the signatures of saved trainsets go with their sceneries

They were kept by the trainset's text for the whole session, behind one lock,
and every reload added more. They are now kept as long as a trainset holds
that text.

StarterNG/Domain/Sceneries/SceneryTrainsetText.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.RegularExpressions;
 using StarterNG.Classes;
@@ -163,17 +164,14 @@ public static class SceneryTrainsetText
     public static bool IsModified(Trainset trainset) =>
         trainset.Parsed && Signature(trainset) != SavedSignature(trainset.RawEntry);
 
-    private static readonly Dictionary<string, string> SavedSignatures = new();
+    /// <summary>
+    /// By the text a trainset holds, for as long as one holds it: a scenery read again
+    /// brings texts of its own, and those of the one it replaces go with it.
+    /// </summary>
+    private static readonly ConditionalWeakTable<string, string> SavedSignatures = new();
 
-    private static string SavedSignature(string raw)
+    private static string SavedSignature(string raw) =>
-    {
+        SavedSignatures.GetValue(raw, text => Signature(new Trainset(text)));
-        lock (SavedSignatures)
-        {
-            if (!SavedSignatures.TryGetValue(raw, out string? signature))
-                SavedSignatures[raw] = signature = Signature(new Trainset(raw));
-            return signature;
-        }
-    }
 
     private static string Signature(Trainset trainset) =>
         trainset.Velocity.ToString(CultureInfo.InvariantCulture) + "\n" +