Draw anim channel colors as a little rectangle in the channel list,
instead of taking over the entire channel name 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.
The channel 'data' background (i.e. the keyframe area) is now drawn with
a consistent color, and much more subtle.
This also enables the 'Channel Group Colors' setting in the preferences
by default, as it is now way less obnoxious and invasive.
Design task: https://projects.blender.org/blender/blender/issues/69059
Reviewed-On: https://projects.blender.org/blender/blender/pulls/112861
c524fbe623