Flight plan fixes based on https://discordapp.com/channels/539048679160676382/539486309882789888/597133111272275970

* improved setting of combox value, fixed wrong text
* do NOT emit signals when same index
* getOwnCallsign
* do NOT used cache FPs for own aircraft

src/blackcore/context/contextownaircraftimpl.h

@@ -81,6 +81,10 @@ namespace BlackCore
             virtual ~CContextOwnAircraft() override;
 
             // IOwnAircraftProvider overrides
+            //! \copydoc BlackMisc::Simulation::IOwnAircraftProvider::getOwnCallsign
+            //! \ingroup ownaircraftprovider
+            virtual BlackMisc::Aviation::CCallsign getOwnCallsign() const override;
+
             //! \copydoc BlackMisc::Simulation::IOwnAircraftProvider::getOwnAircraftPosition
             //! \ingroup ownaircraftprovider
             virtual BlackMisc::Geo::CCoordinateGeodetic getOwnAircraftPosition() const override;