macosx: Fix the preferences' media library folders table

In master, the media library table view in the Preferences window is completely broken, not correctly updating when changes are made and not making the changes to the selected item.

This MR fixes these issues and refactors how the table view is updated.

Screen_Recording_2022-08-22_at_00.49.42

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