Codechange: Refactor timetable GUI (#10613)

src/timetable.h

@@ -13,6 +13,8 @@
 #include "date_type.h"
 #include "vehicle_type.h"
 
+static const uint8 MAX_TIMETABLE_START_YEARS = 15; ///< The maximum start date offset, in years.
+
 void ShowTimetableWindow(const Vehicle *v);
 void UpdateVehicleTimetable(Vehicle *v, bool travelling);
 void SetTimetableParams(int param1, int param2, Ticks ticks);