Fixed bug when undoing layer add

Also created nAppLayers to save the number of layers used by the editor (and that the use can't directly with).

js/_variables.js

@@ -37,4 +37,9 @@ var TMPCanvas = document.getElementById('tmp-canvas');
 // Pixel grid layer
 var pixelGrid;
 // Pixel grid canvas
-var pixelGridCanvas;
+var pixelGridCanvas;
+
+// Index of the first layer the user can use in the layers array
+var firstUserLayerIndex = 2;
+// Number of layers that are only used by the editor
+var nAppLayers = 3;