message: Remove unused signal type
diff: #include "Core/ListBoxBase.h"
  +#include "../ProductCatalogue.h"
  +
  #include <NAS2D/Signal/Signal.h>
  #include <NAS2D/Math/Point.h>
  @@ -18,8 +20,6 @@ class Factory;
  class FactoryListBox : public ListBoxBase
  {
  public:
  - using SelectionChangedSignal = NAS2D::Signal<Factory*>;
  -
  struct FactoryListBoxItem : public ListBoxItem
  {
  FactoryListBoxItem(std::string textDescription, Factory* newFactory, NAS2D::Point<int> iconPosition) :
  