macosx: Clean up and simplify VLCLibraryAudioDataSource

Merged Claudio Cambra requested to merge claucambra/vlc:work/clean-up-audiodatasource into master

By taking advantage of the polymorphic interface provided by the VLCMediaLibraryItemProtocol we can eliminate a lot of code that isn't needed anymore.

This MR also cleans up some other functions in this class that can be made less verbose

Signed-off-by: Claudio Cambra

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