Adding reference circle ( line ) to gears and racks

[J-Dunn]

https://forum.freecadweb.org/viewtopic.php?p=332551#p332551

It would be helpful in aligning gears if the reference circle was visible as a circle ( in a lighter line colour than the gear ) and a similar line was visible for the rack. This is already the reference for placement of the rack part but is not visible in the WB when arranging the parts.