macosx: Play artists' songs correctly from the collection view

This MR adds a new artist-specific collection view item that correctly handles a VLCMediaLibraryArtist object. There have also been changes to the VLCLibraryMenuController, VLCLibraryAudioDataSource, and VLCLibraryDataTypes to correctly support artists and the list of albums under their name

Screen_Recording_2022-06-24_at_00.27.50