Playlist: Remove All watched videos button

[allanlaal]

SHORT Table	(Summary)
Problem	Playlists lack the "Remove Watched" button that Watch Later has
Solution	add that button, make it remove all watched
Alternatives
Scope	Playlist List view
Side effects
Context	uses same delete code as: https://github.com/code-for-charity/YouTube-Extension/issues/636

[allanlaal]

optional: Remove all videos that are >X% watched

(currently Youtubes own Watch Later ´remove watched´ button removes videos even watched for 0.00001 seconds)

[ImprovedTube]

good point! @allanlaal thanks for all your contributions