Draw anim channel colors as a little rectangle in the channel list,
instead of taking over the entire channel background. This keeps the
channel names readable, regardless of the channel colors.
Channel colors are typically set via the bone colors, and since those
are chosen for visual contrast in the 3D Viewport, they aren't
guaranteed to also be a suitable background color for the channel list.
Because of this, it's no longer used as such.
Design task: #69059
