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/Database.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "Metadata.h"
+
+#include <sqlite3.h>
+
+using queue = std::vector<Track>;
+
+class Database {
+    public:
+        Database( const char* filename, sqlite3** db );
+        ~Database();
+        void fetch( const std::string& query );
+        void fetchAlbum( const std::string& album );
+        void addSong( );
+    private:
+        sqlite3* db_;
+        sqlite3_stmt* stmnt;
+};