Copy BaseDifficulty to Beatmap<T> and move all write operations across

osu.Game/Beatmaps/IBeatmap.cs

@@ -20,6 +20,11 @@ namespace osu.Game.Beatmaps
         /// </summary>
         BeatmapMetadata Metadata { get; }
 
+        /// <summary>
+        /// This beatmap's difficulty settings.
+        /// </summary>
+        public BeatmapDifficulty Difficulty { get; set; }
+
         /// <summary>
         /// The control points in this beatmap.
         /// </summary>