Fixes "Add instance" and "Remove instance" hotkeys don't work when object is selected via de object browser #5350

Captures the +- hotkeys in the object list to add / remove an instance.

src/slic3r/GUI/GUI_ObjectList.hpp

@@ -257,6 +257,8 @@ public:
     bool                paste_from_clipboard();
     void                undo();
     void                redo();
+    void                increase_instances();
+    void                decrease_instances();
 
     void                get_settings_choice(const wxString& category_name);
     void                get_freq_settings_choice(const wxString& bundle_name);