flesh out the data scan UI, tweak selection table

Data/Song/Consts.cs

@@ -32,11 +32,6 @@ public static class Consts
     public static readonly IReadOnlyDictionary<uint, string> NUM_SOURCE = _NUM_SOURCE;
     public static readonly IReadOnlyDictionary<string, uint> SOURCE_NUM = _NUM_SOURCE.ToDictionary(p => p.Value, p => p.Key);
 
-    private static string[] _DIFFICULTIES = {
-        "Normal", "Hard", "Expert", "Inferno"
-    };
-    public static readonly IReadOnlyList<string> DIFFICULTIES = _DIFFICULTIES;
-
     private static readonly Dictionary<Difficulty, string> _DIFF_LVL_KEY = new()
     {
         {Difficulty.Normal, "DifficultyNormalLv"},

Data/Song/Song.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Avalonia.Controls;
 using Avalonia.Media;
 using SaturnData.Notation.Core;
 
@@ -21,9 +22,9 @@ public class Song
     public required float PreviewTime { get; set; }
     public required float PreviewLen { get; set; }
     public string SourceName => Consts.NUM_SOURCE[Source];
+    public float?[] Levels { get; set; } = { null, null, null, null };
     
 
     // TODO: For SaturnData.Entry instances, use this Guid format:
     // MERCURY_[SONGID]_[DIFF] (each var is int)
-    public List<(Difficulty, Entry, Chart)> charts = new();
 }