[Feature Request] "Mark video as watched" shortcut

**Is your feature request related to a usage problem (not a bug)? Please describe.**
For the moment we currently need to use the 3-dots menu below a video in order to mark it as watched. That's 2 click on small buttons, so that is not efficient when you need to mark a bunch of video.

**Describe the solution you'd like to see implemented**
It'd be quicker to do it with a button below the thumbnail, or even better with a shortcut (e.g. ALT + LeftClick), so that you just have to click on the thumbnail in the grid/list view.

**Describe alternatives you've considered**
Here is a user script that does that on the Youtube web version (uses a shortcut) : https://greasyfork.org/scripts/30261-mark-watched-youtube-videos/