Support storing files in the iCloud folder

Since local storage can be limited, and working with external drives comes with a whole set of its own problems (like backing those up, for example, traveling with them etc), on macOS it is convenient to use a folder in the iCloud drive for storing catalog assets (not the catalog file itself; that stays local).

macOS allows the user to mark folders as "Keep downloaded", so that when the assets are needed, they can be materialized on disk and C1 can work with them. They can then be offloaded to free up local disk space.

It is already possible to work in this setup, which is what I've been doing for the past ~2.5 years. I've not ran into serious issues, other than C1 displaying a warning sign next to the dirs in iCloud, and sometimes being unable to move large folders from local to iCloud drive from the UI.