Move Length to BeatmapOnlineInfo.

osu.Game/Beatmaps/BeatmapOnlineInfo.cs

@@ -10,6 +10,11 @@ namespace osu.Game.Beatmaps
     /// </summary>
     public class BeatmapOnlineInfo
     {
+        /// <summary>
+        /// The length in milliseconds of this beatmap's song.
+        /// </summary>
+        public double Length { get; set; }
+
         /// <summary>
         /// Whether or not this beatmap has a background video.
         /// </summary>