macosx: Emit the correct notifications in VLCLibraryModel

At the moment, we have several update methods that emit the same notification despite handling different media types/groupings

This MR corrects this and adds a missing notification for Genres

modules/gui/macosx/library/VLCLibraryModel.h

@@ -35,6 +35,7 @@ NS_ASSUME_NONNULL_BEGIN
 extern NSString *VLCLibraryModelAudioMediaListUpdated;
 extern NSString *VLCLibraryModelArtistListUpdated;
 extern NSString *VLCLibraryModelAlbumListUpdated;
+extern NSString *VLCLibraryModelGenreListUpdated;
 extern NSString *VLCLibraryModelVideoMediaListUpdated;
 extern NSString *VLCLibraryModelRecentMediaListUpdated;
 extern NSString *VLCLibraryModelMediaItemUpdated;