feat: detect steps and play audio

app/src/main/cpp/PlaybackEngine.h

@@ -5,13 +5,16 @@
 #ifndef LOCKSTEP_PLAYBACKENGINE_H
 #define LOCKSTEP_PLAYBACKENGINE_H
 
+#include "StepListener.h"
 #include "MixingPlayer.h"
 #include <string>
 
-class PlaybackEngine {
+class PlaybackEngine : public StepListener {
 public:
     PlaybackEngine(std::string filesDir, int resid);
     virtual ~PlaybackEngine();
+    /** Play a beat sound. */
+    virtual void playBeat();
 private:
     MixingPlayer *mPlayer;
     std::string mFilesDir;