add beatmap with storyboard source

osu.Game/Configuration/BackgroundSource.cs

@@ -1,11 +1,16 @@
 // 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.
 
+using System.ComponentModel;
+
 namespace osu.Game.Configuration
 {
     public enum BackgroundSource
     {
         Skin,
-        Beatmap
+        Beatmap,
+
+        [Description("Beatmap (with storyboard / video)")]
+        BeatmapWithStoryboard,
     }
 }