macosx: Deduplicate and make wider use of asynchronous thumbnail fetching
This speeds up initial loads of several data source view items as they no longer have to wait for images to be generated, these are acquired asynchronously.
Also removes a lot of code duplication involving fetching thumbnails by using centralised methods in VLCLibraryImageCache
Signed-off-by: Claudio Cambra developer@claudiocambra.com