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/File.h

@@ -12,7 +12,7 @@ class File {
     public:
         void importFile( std::string fileName );
         void importFolder( const std::string& folderPath, std::vector<Track>& tracks );
-        void readFileTag( const char* fileName );
+        void readFileTag( Track& t );
         std::string parseFiletype( const std::string& path );
 
     private: