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/Plater.hpp

@@ -25,10 +25,13 @@ namespace Slic3r {
 
 class Model;
 class ModelObject;
+class ModelInstance;
 class Print;
 class SLAPrint;
 enum SLAPrintObjectStep : unsigned int;
 
+using ModelInstancePtrs = std::vector<ModelInstance*>;
+
 namespace UndoRedo {
     class Stack;
     struct Snapshot;