Standardize osu!catch coordinate system

There were two coordinate systems used:
- 0..512 (used in osu!stable)
- 0..1 (relative coordinate)

This commit replaces the usage of
the relative coordinate system to
the coordinate system of 0..512.

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

@@ -16,7 +16,16 @@ namespace osu.Game.Rulesets.Catch.UI
 {
     public class CatchPlayfield : ScrollingPlayfield
     {
-        public const float BASE_WIDTH = 512;
+        /// <summary>
+        /// The width of the playfield.
+        /// The horizontal movement of the catcher is confined in the area of this width.
+        /// </summary>
+        public const float WIDTH = 512;
+
+        /// <summary>
+        /// The center position of the playfield.
+        /// </summary>
+        public const float CENTER_X = WIDTH / 2;
 
         internal readonly CatcherArea CatcherArea;