[Feature Request] Implement a "Watched" Playlist System

[SakiSakiSakiSakiSaki]

Since "Watch Later" and "History" don't produce links, and require User Auth, you can probably implement a scuffed workaround.

Allow a user to designate a manually created playlist as their "Watch Later" playlist, (I named mine "Watch Soon").
Allow them to designate a manually created playlist as their "History" playlist, (I made mine "Watched".)

The idea is that when a user is done watching a video, they can remove it from their "Watch Soon" list and add it to their "Watched" list. Videos in the "Watched" list will be removed from the "Watch Soon" list.

Another method of "cleaning up" watched videos without accidentally removing deleted videos, is to check to see if a video that is missing in a playlist suddenly is still on YouTube.
If Yes, then remove from hard drive and where ever else.
If No, then keep it on your hard drive.

Downside of this is that you have to keep deleted videos on your YouTube frontend playlist if you don't want to lose them on your hard drive. But perhaps you can make this optional on a playlist-by-playlist basis, such that I could only enable this method on the "Watch Soon" playlist.