refs #768, improved selectors

* added focus proxy
* using &QLineEdit::editingFinished as signal (changes value also when focus is lost)
* use enum to set how values are displayed
* search for DB values by leveraging the smart selector functions
* added isSet functions
* minor renamings/formatting

src/blackgui/components/dbairlineicaoselectorbase.h

@@ -61,6 +61,9 @@ namespace BlackGui
             //! Clear selection
             virtual void clear() = 0;
 
+            //! Set with valid value
+            bool isSet() const;
+
         signals:
             //! ICAO was changed
             void changedAirlineIcao(const BlackMisc::Aviation::CAirlineIcaoCode &icao);