Reorganized file tree, made Checkerboard a child of Layer

js/data/consts.js

@@ -0,0 +1,9 @@
+const MIN_Z_INDEX = -5000;
+const MAX_Z_INDEX = 5000;
+
+// Index of the first layer the user can use in the layers array
+const firstUserLayerIndex = 2;
+// Number of layers that are only used by the editor
+const nAppLayers = 3;
+
+const MIN_ZOOM_LEVEL = 0.5;