Setting poster image for videos inside a video playlist is not very intuitive

[Fab1en]

This is somehow a follow-up to #27891.
When you create a video playlist, it's very hard to understand how to change the poster images of the videos.

Here is the workflow :

1. Upload some videos
2. Create a new article and insert one single video
3. Edit the inserted video to open the media modal again, and select a poster image for your video
4. Nice !
5. Create another article and insert a video playlist
6. When you choose video, the one that was inserted in the previous article is shown with the poster image instead of the default video icon : very nice !
7. For other videos, there is no way to select a poster image, neither during initial playlist creation, nor after having inserted the playlist :-(
8. To select a poster image, you have to go back to the main Media menu, find the right video and edit its details, and then set the featured image (that you guessed it was the same as the poster image)

I see at least three ways to improve that workflow.

1. In the "Edit video playlist" modal, add a "Select poster image" menu, similar to the one that exists in the "Video details" modal for the video shortcode.
2. When browsing videos in the media modal, add a link to set the poster image, in the same manner that the "Edit image" link works for images
3. When browsing videos in the main Medias menu, add a link to set the poster image, in the same manner that the "Edit image" link works for images
4. Bonus : change "Featured Image" title to "Poster image" in the video attachment post edition page
5. Big Bonus : when browsing videos in the main Medias menu, add a button "take a screenshot and set as poster" when a video is shown. This will load the video into a canvas, generate an image from this canvas, send the image to the server and set it as poster !

[wonderboymusic]

I may pick this up