macosx: Add a supplementary view for genres and artists collection views

Similarly to the album supplementary views in the collection view, this adds collapsible supplementary views for genres and artists. These use the same views as the library selection table view.

Screen_Recording_2022-08-13_at_18.17.40

Screen_Recording_2022-08-13_at_18.11.31

Signed-off-by: Claudio Cambra claudio.cambra@gmail.com