Add departure boards and timetable tick status to SLXI info.

Implementing converting timetable start date to/from ticks/days in
afterload code.

src/saveload/extended_ver_sl.h

@@ -21,6 +21,8 @@
  */
 enum SlXvFeatureIndex {
 	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
+	XSLFI_DEPARTURE_BOARDS,                       ///< Departure boards patch, in ticks mode
+	XSLFI_TIMETABLES_START_TICKS,                 ///< Timetable start time is in ticks, instead of days (from departure boards patch)
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };