Improve pawn promotion code

src/game_tab/left_panel/board/BoardCanvas.hpp

@@ -12,6 +12,7 @@
 
 // Local events
 wxDECLARE_EVENT(PLAY_MOVE_EVENT, wxCommandEvent);
+wxDECLARE_EVENT(PLAY_PROMOTE, wxCommandEvent);
 
 #define REFRESH_MOUSE_LOCATION()                                               \
   {                                                                            \
@@ -74,6 +75,7 @@ typedef struct GameState {
   } Square;
   std::string white, black;
   std::string board;
+  std::string promotion;
   std::map<char, std::uint8_t> captures;
   std::vector<Square> squares_hl;
   std::vector<Arrow> arrows;