Ref T385, OBS aircraft callsign

* in CAirspaceMonitor::onIcaoCodesReceived we know we deal with aircraft only and explicitly deal with it
* in other cases we can not tell if we deal with aircraft/ATC callsign for sure, we ignore invalid callsigns

src/blackmisc/aviation/callsign.h

@@ -84,6 +84,9 @@ namespace BlackMisc
             //! Supervisor?
             bool isSupervisorCallsign() const;
 
+            //! Pilot OBS callsign, normally a co-pilot
+            bool isCopilotCallsign() const;
+
             //! Get callsign (normalized)
             const QString &asString() const { return m_callsign; }