refs #372 Add audio mixer interface and vatlib implementation

This class is an audio router. In order to route audio connect a
producer to an input port and a consumer to an output port. The
connection is later established by making the mixer connection
between an input and an output port.

src/blackcore/voice_vatlib.cpp

@@ -6,6 +6,7 @@
 #include "voice_vatlib.h"
 #include "voice_channel_vatlib.h"
 #include "audio_device_vatlib.h"
+#include "audio_mixer_vatlib.h"
 #include "blackmisc/logmessage.h"
 #include "blackmisc/blackmiscfreefunctions.h"
 #include <QDebug>
@@ -53,6 +54,11 @@ namespace BlackCore
         return make_unique<CAudioOutputDeviceVatlib>(m_audioService.data(), this);
     }
 
+    std::unique_ptr<IAudioMixer> CVoiceVatlib::createAudioMixer()
+    {
+        return make_unique<CAudioMixerVatlib>(this);
+    }
+
     /* FIXME:
        Can the following methods be more general somehow?
        E.g.: