Add the concept of nested DrawableHitObjects.

- Applies to Slider Ticks and start circle. repeat/endpoints still need addressing.
- Removed SliderTicksLayer abstraction for now.

osu.Game.Modes.Osu/Objects/Drawables/DrawableOsuHitObject.cs

@@ -73,5 +73,7 @@ namespace osu.Game.Modes.Osu.Objects.Drawables
         Hit100,
         [Description(@"300")]
         Hit300,
+        [Description(@"10")]
+        SliderTick
     }
 }