added Selected_instance field to Plater2DObject, also added reference to object index to Plater.

src/GUI/Plater.hpp

@@ -21,6 +21,7 @@
 namespace Slic3r { namespace GUI {
 
 using UndoOperation = int;
+using obj_index = unsigned int;
 
 class Plater2DObject;
 class Plate2D;

src/GUI/Plater/Plater2DObject.hpp

@@ -8,6 +8,7 @@ public:
     std::string name {""};
     std::string identifier {""};
     bool selected {false};
+    int selected_instance {-1};
 };
 } } // Namespace Slic3r::GUI
 #endif