Updated ScenePartitionEditorWindow

- Added clear button here instead of in ScenePartitionSOEditor
- Added editor util for horizontal line
- Exposed scene grid cell size value

Runtime/SceneGrid.cs

@@ -6,7 +6,7 @@ namespace VertexColor.ScenePartition
     public class SceneGrid
     {
         [SerializeField]
-        private int cellSize = 10;
+        public int cellSize = 10;
 
         [SerializeField]
         private SceneGridDictionary grid = new SceneGridDictionary();
@@ -81,4 +81,4 @@ namespace VertexColor.ScenePartition
             return (x, y);
         }
     }
-}
+}