Add chapters display, gray out fixed tracks, spacebar preview

Models/MkvModels.cs

@@ -13,6 +13,15 @@ public class MkvIdentifyResult
 
     [JsonPropertyName("tracks")]
     public List<MkvTrack> Tracks { get; set; } = new();
+
+    [JsonPropertyName("chapters")]
+    public List<MkvChapters> Chapters { get; set; } = new();
+}
+
+public class MkvChapters
+{
+    [JsonPropertyName("num_entries")]
+    public int NumEntries { get; set; }
 }
 
 public class MkvTrack