Swallow GtkHSV back into GIMP and call it GimpColorWheel. Keep it in
modules/ for the time being. Clean up and undeprecate it. Replace
set_metrics() API by set_ring_fraction() and make it follow the size
of its parent container, making its use straightforward. Kept it clean
of GIMP color types so it can be easily adapted by e.g. Inkscape.