Refactor tagging and add library scanning

- Replace `Tag` class with utility helpers for hashing and file type parsing
- Change `Track.artwork` from `std::string` to `TagLib::VariantMap`
- Add `importArtwork`, rename `importFolder` to `scanFolder`
- Add `scanLibrary`, `updateLibrary`, `editMetadata`, and `seek` methods to `Player` hierarchy
- Remove obsolete `Tag.h`/`Tag.cpp` files

include/File.h

@@ -11,8 +11,9 @@ namespace fs = std::filesystem;
 class File {
     public:
         Track importFile( const std::string& filePath );
-        void importFolder( const std::string& folderPath, std::vector<Track>& tracks );
+        void scanFolder( const std::string& folderPath, std::vector<Track>& tracks );
         void readFileTag( Track& t );
+        TagLib::VariantMap importArtwork( Track& t );
         std::string parseFiletype( const std::string& path );
         void updateLibrary();
         uint64_t hashString( Track t );