Blender Git Loki

Add a new B-Bone Custom Handle type that uses the handle bone direction.

Both original handle types are based on location, and Absolute uses it
in a weird way: the Start handle uses the head, while End uses the tail.

This makes controlling the shape of the B-Bone via control bone rotation
really non-intuitive, especially if trying to add a single control for
the tangent in the middle of a B-Bone chain.

To remedy this, add a new custom handle type that uses the orientation
of the control bone, while completely ignoring location. It is even
possible to control both ends of one B-Bone with the same handle bone,
resulting in an S shape.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D3769