Get current callsign as observer callsign.

This will be used with OBS mode login.

src/blackmisc/avcallsign.cpp

@@ -41,6 +41,18 @@ namespace BlackMisc
             return unified;
         }
 
+        /*
+         * Callsign as Observer
+         */
+        QString CCallsign::getAsObserverCallsignString() const
+        {
+            if (this->isEmpty()) return "";
+            QString obs = this->getStringAsSet();
+            if (obs.endsWith("_OBS", Qt::CaseInsensitive)) return obs;
+            if (obs.contains('_')) obs = obs.left(obs.lastIndexOf('_'));
+            return obs.append("_OBS").toUpper();
+        }
+
         /*
          * Equals callsign?
          */