This PR adds a flag to the pose bone that determines its visibility.
Doing so allows to hide a pose bone and
* don't affect other instances of the same armature
* save the visibility even if the rig is linked
The visibility of the bone in object mode is now also determined by the
pose bone whereas before it was the `Bone`.
**This breaks backwards compatibility** on the Python API since the visibility property,
on the `Bone` behaves differently now as it hides the edit bone instead of the pose bone.
In order to remove all active uses of `BONE_HIDDEN_P` the changes in `armature_skinning.cc` are required.
Part of #138482
Pull Request: https://projects.blender.org/blender/blender/pulls/139167
a43359eb88