Add modifier key window for toggling shift/ctrl key states using mouse

src/gui.h

@@ -59,6 +59,8 @@ void ShowEstimatedCostOrIncome(Money cost, int x, int y);
 void ShowExtraViewPortWindow(TileIndex tile = INVALID_TILE);
 void ShowExtraViewPortWindowForTileUnderCursor();
 
+void ShowModifierKeyToggleWindow();
+
 /* bridge_gui.cpp */
 void ShowBuildBridgeWindow(TileIndex start, TileIndex end, TransportType transport_type, byte bridge_type);