Allow choosing Collection art from Collection's movies

I know there’s a suggestion for Collection scraping but I want to propose something probably much simpler that can be very beneficial for Collection management (I hope this is not a duplicate).

The idea is that in the Collection’s Edit modal window, it would be nice if you could pick both Posters and Background from the movives that belong to the Collection and are already scraped and potentially contain both Posters and Backgrounds.

The Collection art “pool” would be kept updated every time a new movie is added.

Early 2021 clean-up: this is a feature suggestion for a workaround of a “far fetched feature” that has been implemented