Split out individual pieces into own files

osu.Game/Beatmaps/BeatmapSetOnlineLanguage.cs

@@ -0,0 +1,11 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Beatmaps
+{
+    public class BeatmapSetOnlineLanguage
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+    }
+}