[xswiftbus] Replace QtDBus with a libevent driven C++ wrapper on top of libdbus

QtDBus was the main component of xswiftbus' Qt dependency. This is the
first preparation step to get xswiftbus Qt free.
The new implementation is based on the low level libdbus library, which
was also in use by QtDBus itself. But instead of QtDBus, we use now a thin
C++ wrapper. To keep DBus handling async, libevent is used to monitor
timeouts and fds.

src/plugins/simulator/xplane/xswiftbustrafficproxy.h

@@ -58,11 +58,11 @@ namespace BlackSimPlugin
             bool isValid() const { return m_dbusInterface->isValid(); }
 
         signals:
-            //! \copydoc XSwiftBus::CTraffic::simFrame
+            //! Simulator frame
             //! \remark from simulator to driver
             void simFrame();
 
-            //! \copydoc XSwiftBus::CTraffic::remoteAircraftData
+            //! Remote aircraft data
             //! \remark from simulator to driver for elevation and CG
             void remoteAircraftData(const QString &callsign, double latitudeDeg, double longitudeDeg, double elevationMeters, double modelVerticalOffsetMeters);