Refactor WorkingBeatmap

Gets ArchiveReader out of WorkingBeatmap and delegates extracting stuff
from it to subclasses. Should enable us to make an OnlineWorkingBeatmap
or so.

osu.Desktop.VisualTests/Beatmaps/TestWorkingBeatmap.cs

@@ -0,0 +1,32 @@
+using System;
+using osu.Framework.Audio.Track;
+using osu.Framework.Graphics.Textures;
+using osu.Game.Beatmaps;
+using osu.Game.Beatmaps.IO;
+
+namespace osu.Desktop.VisualTests.Beatmaps
+{
+    public class TestWorkingBeatmap : WorkingBeatmap
+    {
+        public TestWorkingBeatmap(Beatmap beatmap)
+            : base(beatmap.BeatmapInfo, beatmap.BeatmapInfo.BeatmapSet)
+        {
+            this.beatmap = beatmap;
+        }
+
+        private Beatmap beatmap;
+        public override Beatmap Beatmap => beatmap;
+        public override Texture Background => null;
+        public override Track Track => null;
+        
+        public override void Dispose()
+        {
+            // This space intentionally left blank
+        }
+        
+        public override void TransferTo(WorkingBeatmap other)
+        {
+            // This space intentionally left blank
+        }
+    }
+}