Improved stability (fixed crashes) of the Cut by plane function by

replacing the cut triangulation with freeglu tesselator.
Added performance tracing output of the Cut by plane function.
Added wait cursor to split to parts / objects, object cut, save to AMF/3MF.

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -1347,6 +1347,8 @@ void ObjectList::split()
         return;
     }
 
+    wxBusyCursor wait;
+
     auto model_object = (*m_objects)[obj_idx];
 
     auto parent = m_objects_model->GetTopParent(item);