The idea is inspired by similar shapes in other rigs like BlenRig.
The actual used shape is simply a circle deformed into a parabolic
shape when viewed from the side, added to create_circle_widget.

Using the new shape for the main controls allows adding extra FK
controls to the spine without visual confusion.

As an aside, fix a minor issue in handling bone_transform_name,
and remove redundant bone_transform_name=None parameters.