[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/xswiftbusserviceproxy.h

@@ -85,12 +85,12 @@ namespace BlackSimPlugin
             //! Emitted if an asynchronous method call caused a DBus error
             BLACK_NO_RELAY void asyncMethodError(QDBusError error);
 
-            //! \copydoc XSwiftBus::CService::aircraftModelChanged
+            //! Own aircraft model changed
             void aircraftModelChanged(
                 const QString &path, const QString &filename, const QString &livery,
                 const QString &icao, const QString &modelString, const QString &name, const QString &description);
 
-            //! \copydoc XSwiftBus::CService::airportsInRangeUpdated
+            //! Airports in range are updated
             void airportsInRangeUpdated(const QStringList &icaoCodes, const QStringList &names, const QList<double> &lats, const QList<double> &lons, const QList<double> &alts);
 
         public slots: