Categories edition/creation: list all colors already used
Previously the categories creation/edition was done on the same screen as the categories listing, which allowed to see all the colors already being used and thus to choose a distinct color.
Now that the creation/edition is done on a new screen we should list the colors which are already being used so that the user can pick another one easily. Maybe through a list of small colored squares, directly on the screen since we have the space, but a popover would be fine too I guess…