Comment 10 for bug 872006

This would be incredibly handy.

Rekordbox has had the ability to select the deck for their traffic light indicator system since 2018; https://www.youtube.com/watch?v=26C6MowxSy0

Though, as Phil Morse points out and breaks down in their really good CDJ-3000 review - https://youtu.be/9eECvYYAwbg?t=2370 - there are really six options to choose from, not just four, though covering that range in Pioneer gear is apparently a new thing.

(An alternative option to having it in the library could be a UI feature in a similar shape/location as the Spinnies but that would indeed be an advanced faff that would need a lot of thinking out to get right. Just colouring the Key column fields and having a dropdown for current/deck1/deck2/deck3/deck4, as a straight copy of other implementations, would seem most sensible.)

((Also, is it not "Open Key" rather than "OpenKey"? Full name "Open Key Notation". Even the graphic from the Mixxx blog post about it has the space.))