[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/dbuserror.cpp

@@ -0,0 +1,19 @@
+/* Copyright (C) 2018
+ * swift project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
+ * including this file, may be copied, modified, propagated, or distributed except according to the terms
+ * contained in the LICENSE file.
+ */
+
+#include "dbuserror.h"
+
+namespace XSwiftBus
+{
+
+    CDBusError::CDBusError(const DBusError *error)
+        : m_name(error->name), m_message(error->message)
+    { }
+
+}