Watchlist support for episodes and seasons in V3

The recently completed feature for adding episodes and seasons to lists (https://roadmap.trakt.tv/p/adding-episodes-and-seasons-to-lists-in-v3) does not cover adding episodes and seasons to the watchlist, which is still missing in V3. However, it is still missing the crucial function of adding episodes and seasons to your watchlist. The watchlist needs the same ability to add episodes and seasons, as the originally-requested feature is not fully completed yet and more work is to be done.