In addition to the original bug report, I've gone through cleaning up a range of
related bugs which only became clear when hunting around the code...
* Custom Handle References weren't getting cleared when the bones they used got
deleted. But, neither was the custom bone shape location/transform reference.
* Various places where posebone settings are copied around were also missing code
to handle the new Bendy Bone properties.
(WHY DO WE HAVE SO MANY VARIATIONS OF COPYING POSE DATA!?!?)
* If duplicating a Bendy Bone with custom references, and the custom references
are also selected/duplicated, the new Bendy Bones will use the corresponding
duplicated bones
6e416b6bdf