message: Use struct for StructureListBoxItem
diff: @@ -19,7 +19,7 @@ public:
  using SelectionChangedCallback = NAS2D::Signals::Signal<Structure*>;
  public:
  - class StructureListBoxItem : public ListBoxItem
  + struct StructureListBoxItem : public ListBoxItem
  {
  public:
  StructureListBoxItem(Structure* s);
  