Refactor SECLAL player into new threaded player class

The reason for moving the implementation out from CSoundGenerator into
its own class is, because CSoundGenerator was a very complex and
obscure class. It mixed many tasks in one place. CSelcalPlayer
is designed to play SELCALs only.

The following design changes have been made, compared to CSoundGenerator:
* Use pull mode instead of push mode. QBuffer is used as the QIODevice and
  is a wrapper around QByteArray. Therefore it is not necessary to
  implement our own QIODevice.
* Internally it uses a CThreadedSelcalPlayer to relieve the load of the
  main thread. CThreadedSelcalPlayer inherits CContinuousWorker, no
  low level QThread implementation was necessary.
* Push mode was not implemented.
* It is important that the QAudioOutput is allocated in the worker thread.
  QAudioOutput allocates internal objects, which cannot be moved to
  the worker thread.
* Data caching. The generated seclal audio data is cached.

refs #736

src/blackcore/context/contextaudioimpl.h

@@ -28,6 +28,7 @@
 #include "blackmisc/identifier.h"
 #include "blackmisc/network/userlist.h"
 #include "blackmisc/settingscache.h"
+#include "blacksound/selcalplayer.h"
 
 #include <QHash>
 #include <QList>
@@ -181,6 +182,7 @@ namespace BlackCore
 
             QList<QSharedPointer<IVoiceChannel>> m_unusedVoiceChannels;
             QHash<BlackMisc::Aviation::CComSystem::ComUnit, QSharedPointer<IVoiceChannel>> m_voiceChannelMapping;
+            BlackSound::CSelcalPlayer *m_selcalPlayer = nullptr;
 
             // settings
             BlackMisc::CSetting<BlackCore::Audio::TSettings> m_audioSettings { this };