This PR changes the uses of `PBONE_SELECTED` and all the
places where it *should* have been used
with a new function `bone_is_selected` or `...editbone`
and `...pose_bone` specializations.
No functional changes intended.
Do note that there are still places in the code
where this function should probably be called,
but this PR is very careful not to change any behavior,
even if the current behavior is probably wrong.
In preparation for storing pose bone selection state
on the `bPoseChannel`
Related to #138482
Pull Request: https://projects.blender.org/blender/blender/pulls/139496
5dfbe1af21