Includes cleanup:

GUI_App.hpp      (should not include) MainFrame.hpp
MainFrame.hpp    Plater.hpp
ModelArrange.hpp Model.hpp
Slicing.hpp      PrintConfig.hpp
FillBase.hpp     PrintConfig.hpp
GUI_App.hpp      PrintConfig.hpp
OptionsGroup.hpp GUI_App.hpp

src/slic3r/GUI/GUI_ObjectLayers.cpp

@@ -2,9 +2,11 @@
 #include "GUI_ObjectList.hpp"
 
 #include "OptionsGroup.hpp"
+#include "GUI_App.hpp"
 #include "PresetBundle.hpp"
 #include "libslic3r/Model.hpp"
 #include "GLCanvas3D.hpp"
+#include "Plater.hpp"
 
 #include <boost/algorithm/string.hpp>