Allow arbitrary height catch editor

Useful because can see more notes at once.

osu.Game.Rulesets.Catch/UI/CatchPlayfield.cs

@@ -23,6 +23,12 @@ namespace osu.Game.Rulesets.Catch.UI
         /// </summary>
         public const float WIDTH = 512;
 
+        /// <summary>
+        /// The height of the playfield.
+        /// This doesn't include the catcher area.
+        /// </summary>
+        public const float HEIGHT = 384;
+
         /// <summary>
         /// The center position of the playfield.
         /// </summary>