Add Database and refactor player/playback

Add Database class with SQLite fetch/fetchAlbum methods. Refactor
PlaybackEngine (data callback, seek, getPosition, decoder lifecycle)
and Player API (PlaybackEngine member, queue handling, PlayerLocal).
Update Metadata::Track fields to include artist/title/album/trackNr/length.

include/PlayerLocal.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "Player.h"
+
+class PlayerLocal : public Player {
+  public:
+
+    void next() override;
+    void previous() override;
+    ma_uint64 getPlayheadPosition () override;
+    void loadFile ( Track track ) override;
+    void addToQueue( Track& track );
+    void playQueue();
+
+  private:
+      unsigned int currentTrack_;
+};