Add flag to toggle follow circle tracking for slider heads

osu.Game.Rulesets.Osu/Objects/SliderHeadCircle.cs

@@ -5,5 +5,9 @@ namespace osu.Game.Rulesets.Osu.Objects
 {
     public class SliderHeadCircle : HitCircle
     {
+        /// <summary>
+        /// Makes the head circle track the follow circle when the start time is reached.
+        /// </summary>
+        public bool TrackFollowCircle = true;
     }
 }