[videodatabase] adds GetTvShowSeasons
[videodatabase] move AddSeason() public.
[scrapers] adds GetArt function to (video) scraper, allowing art to be fetched given the video identifier.
[videoscanner] refresh season art if a new season is found that isn't recorded in the database yet. Fixes #14339
REMOVEME: updated thetvdb.com scraper to support art updates