Blender Git Loki

Bendy Bones: Added ability to add custom reference prev/next bones for controlling handles

This is an experimental option which makes it possible to specify which bone
to use as the reference handle for the previous/next bones, instead of only
using those that are directly connected on either end.

To use:
* Enable the "Use Custom Handle References" option in the Bendy Bones panel
* Set Start and/or End bones accordingly. If both are left blank, then the BBone
will only respond to whatever offsets have been set by the animator
* Be careful when positioning the start/end bones! It will use the bone position
as the point at which the handle currently sits - best results seem be when these
bones are in line with the bbone to start with.

Why (according to @jpbouza):
When you have a very long chain of bbones, as they are all parented, when you rotate
the first bone in the chain (Y rotation, like twisting the chain), all the child
bones rotate. So the only way to avoid this is to add a copy rotation constraint to
each bone in the chain in order to override their parent rotation, thus achieving
control over twisting of each segment of the chain.

This commit makes it possible to have a bone (that is not a parent or child of the bone
being affected) to influence the bbone curvature as if they were.