Ref T129, some more utility functions for aviation value objects

* removed static variables from SELCAL
* more finder functions for airline ICAO list
* check for IATA code
* unify callsign now public

src/blackmisc/aviation/airlineicaocode.h

@@ -228,6 +228,9 @@ namespace BlackMisc
             //! Valid designator?
             static bool isValidAirlineDesignator(const QString &airline);
 
+            //! Valid IATA code?
+            static bool isValidIataCode(const QString &iataCode);
+
             //! Some special valid designator which do not fit standard rule (e.g. 3-letter code)
             static QSet<QString> specialValidDesignators();