Include cleanup: Selection.hpp

It does not need 3DScene.hpp and Model.hpp
And it does not to be included by GLGizmoBase.hpp

src/slic3r/GUI/GLCanvas3D.hpp

@@ -42,6 +42,9 @@ struct ThumbnailData;
 class ModelObject;
 class ModelInstance;
 class PrintObject;
+class Print;
+class SLAPrint;
+namespace CustomGCode { struct Item; }
 
 namespace GUI {