[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/xswiftbus/weather.h

@@ -15,8 +15,8 @@
 #ifndef NOMINMAX
 #define NOMINMAX
 #endif
+#include "dbusobject.h"
 #include "datarefs.h"
-#include <QObject>
 
 //! \cond PRIVATE
 #define XSWIFTBUS_WEATHER_INTERFACENAME "org.swift_project.xswiftbus.weather"
@@ -29,30 +29,26 @@ namespace XSwiftBus
     /*!
      * XSwiftBus weather object which is accessible through DBus
      */
-    class CWeather : public QObject
+    class CWeather : public CDBusObject
     {
-        Q_OBJECT
-        Q_CLASSINFO("D-Bus Interface", XSWIFTBUS_WEATHER_INTERFACENAME)
-
     public:
         //! Constructor
-        CWeather(QObject *parent) : QObject(parent) {}
+        CWeather(CDBusConnection *dbusConnection);
 
         //! DBus interface name
-        static const QString &InterfaceName()
+        static const std::string &InterfaceName()
         {
-            static QString s(XSWIFTBUS_WEATHER_INTERFACENAME);
+            static std::string s(XSWIFTBUS_WEATHER_INTERFACENAME);
             return s;
         }
 
         //! DBus object path
-        static const QString &ObjectPath()
+        static const std::string &ObjectPath()
         {
-            static QString s(XSWIFTBUS_WEATHER_OBJECTPATH);
+            static std::string s(XSWIFTBUS_WEATHER_OBJECTPATH);
             return s;
         }
 
-    public slots:
         //! True if the sim is using X-Plane built-in real weather source.
         bool isUsingRealWeather() const { return m_useRealWeather.get(); }
 
@@ -101,6 +97,10 @@ namespace XSwiftBus
         //! \param turbulence Amount of turbulence [0,10].
         void setWindLayer(int layer, int altitude, double direction, int speed, int shearDirection, int shearSpeed, int turbulence);
 
+        virtual int processDBus() override;
+    protected:
+        virtual DBusHandlerResult dbusMessageHandler(const CDBusMessage &message) override;
+
     private:
         DataRef<xplane::data::sim::weather::use_real_weather_bool> m_useRealWeather;
         DataRef<xplane::data::sim::weather::visibility_reported_m> m_visibilityM;