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/libslic3r/ModelArrange.hpp

@@ -1,11 +1,14 @@
 #ifndef MODELARRANGE_HPP
 #define MODELARRANGE_HPP
 
-#include <libslic3r/Model.hpp>
 #include <libslic3r/Arrange.hpp>
 
 namespace Slic3r {
 
+class Model;
+class ModelInstance;
+using ModelInstancePtrs = std::vector<ModelInstance*>;
+
 using arrangement::ArrangePolygon;
 using arrangement::ArrangePolygons;
 using arrangement::ArrangeParams;