Blender Git Loki

Fix T48470: Bendy Bones: Custom Handle References not being cleared when bone deleted

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