implemented preliminary pre-cacher for audio

FFmpegDecoder now implements an "indexer" for audio, which actually just
extracts the raw PCM audio from an audio codec and saves it to disk

app/decoder/wave.h

@@ -0,0 +1,39 @@
+#ifndef WAVEAUDIO_H
+#define WAVEAUDIO_H
+
+#include <QByteArray>
+#include <QFile>
+
+#include "audio/sampleformat.h"
+#include "render/audio/audioparams.h"
+
+class WaveOutput
+{
+public:
+  WaveOutput(const QString& f,
+             const AudioRenderingParams& params);
+
+  ~WaveOutput();
+
+  bool open();
+
+  void write(const QByteArray& bytes);
+  void write(const char* bytes, int length);
+
+  void close();
+
+private:
+  template<typename T>
+  void write_int(QFile* file, T integer);
+
+  void switch_endianness(QByteArray &array);
+
+  QFile file_;
+
+  AudioRenderingParams params_;
+
+  int data_length_;
+
+};
+
+#endif // WAVEAUDIO_H