Implemented ObjectLayers class

+ some code refactoring

src/slic3r/GUI/GUI_App.hpp

@@ -155,6 +155,7 @@ public:
     ObjectManipulation* obj_manipul();
     ObjectSettings*     obj_settings();
     ObjectList*         obj_list();
+    ObjectLayers*       obj_layers();
     Plater*             plater();
     std::vector<ModelObject*> *model_objects();