message: Add Callback to SelectionChanged type name
  Using Callback is perhaps not a great name, but being consistent will at
  least make global search/replace easier when we want to choose a better
  name.
diff: class ComboBox : public Control
  {
  public:
  - using SelectionChanged = NAS2D::Signal<>;
  + using SelectionChangedCallback = NAS2D::Signal<>;
  ComboBox();
  ~ComboBox() override;
  @@ -26,7 +26,7 @@ public:
  void clearSelected();
  - SelectionChanged& selectionChanged() { return mSelectionChanged; }
  + SelectionChangedCallback& selectionChanged() { return mSelectionChanged; }
  const std::string& selectionText() const;
  int selectionTag() const;
  @@ -54,7 +54,7 @@ private:
  NAS2D::Rectangle<int> mBaseArea;
  - SelectionChanged mSelectionChanged;
  + SelectionChangedCallback mSelectionChanged;
  std::size_t mMaxDisplayItems = constants::MINIMUM_DISPLAY_ITEMS;
  };
  