Change default beat divisor to 4

This matches stable, and is a much saner default value.

Will apply to new beatmaps and also beatmaps which don't specify a snap
value in the `.osu` file.

osu.Game/Beatmaps/BeatmapInfo.cs

@@ -167,7 +167,7 @@ namespace osu.Game.Beatmaps
         /// </remarks>
         public double DistanceSpacing { get; set; } = 1.0;
 
-        public int BeatDivisor { get; set; }
+        public int BeatDivisor { get; set; } = 4;
 
         public int GridSize { get; set; }