macosx: Update media library item data sources granularly

At the moment, we update data sources by brute-force, reloading the entirety of the views and the data arrays. This causes frequent slowness in the app whenever as well as flickering in the view items.

By updating data sources more intelligently, we ensure that we still have an up to date representation of the media library without these issues

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