[Feature] Automatically Set Art (Shortcut icon, Background, Cover, etc.) for Flatpaks when using Add to steam

[thegreatGreenstar]

While this feature provides a clear and direct benefit to SteamOs because of its flatpak usage, it is most relevant to Steam for linux.

When a Flatpak is right clicked, there is an option (on SteamOs at least) to add it to steam. However when this is done, it has a blank cover, icon, etc. However, this doesn't need to be this way. Because Flatpaks from Flathub are forced to have art/images which can fulfill most of these automatically. For example with the Flatpak appplication Sober:

The Icon is obvious, while the rest will use the closest fitting to optimal resolution screenshots/images. For more visuals, this should look something like:

While not a perfect example, I believe this gets the point across. As for the actual implementation, this can use the Flathub API: https://flathub.org/api/v2/docs