Update Track fields and file tag handling

Change Track members: artwork -> std::string, genre -> TagLib::String, and add year
File::readFileTag now takes a Track& and fills artist/title/album/trackNr/year/genre/format/length
importFolder sets artwork to <folder>/cover.png
Rename Database::addSong to addTrack

include/Database.h

@@ -13,7 +13,7 @@ class Database {
         void fetch( const std::string& query ) const;
         void fetchAlbum( const std::string& album ) const;
         void fetchAll() const;
-        void addSong( );
+        void addTrack();
     private:
         sqlite3* db_;
         sqlite3_stmt* stmnt;