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/Database.h

@@ -22,10 +22,12 @@ class Database {
         std::optional<int> getArtistId( const std::string& artistName );
         std::optional<int> getAlbumId( const std::string& albumTitle );
         std::optional<int> getGenreId( const std::string& genre );
+
         void addTrack( Track t);
         void addArtist( const std::string& artistName );
         void addAlbum( Track t);
-        void addGenre( std::string& genre );
+        void addGenre( const std::string& genre );
+
         void setActive( bool active );
         void updateFilePath( const std::string& filePath );