Added LayerList to handle all the functions that involve more than 1 layer

Merge, flatten, add layer, delete layer etc, I'm planning to move a few global variables in Layer to LayerList.

js/pixel-editor.js

@@ -13,7 +13,8 @@
 /**init**/
 //=include _consts.js
 //=include _settings.js
-//=include _layer.js
+//=include LayerList.js
+//=include Layer.js
 //=include Startup.js
 //=include _pixelGrid.js
 //=include _editorMode.js