macosx: Add a header view for audio groups in library, refactor VLCLibraryAudioGroupDataSource

This MR adds a header view to the collection view in the grid-mode audio library group sections.

As a consequence, this MR has a large refactoring of the VLCLibraryAudioGroupDataSourceto properly separate the duties of it from VLCLibraryAudioDataSource. This code should now be less spaghetti-like, and helps make the implementation of the audio header view simpler.

Signed-off-by: Claudio Cambra developer@claudiocambra.com