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/Containers.h

@@ -29,9 +29,10 @@ struct Track {
     TagLib::String album;
     unsigned int trackNr;
     unsigned int length;
+    unsigned int year;
     std::string filePath;
-    std::string_view artwork;
-    std::string_view genre;
+    std::string artwork;
+    TagLib::String genre;
     std::string format;
 };