multiple playlists now work as intended! this means that…

• if a song is present in multiple playlists, it will only download the song once
• if a song is removed from one playlist, it will not remove it if it is present in another one
• if a song is removed from ALL playlists, it will be removed

this meant i had to do a little bit of refactoring, before i was storing a map of the playlist id with a vector of strings, now its just a vector of video ids, which made this a lot easier to implement!