This is a request from the animators at BI.
They wanted to be able to use the X-axis mirror in pose mode. With an option to use relative positioning or absolute positioning of the mirror.
Here is my initial take on it.
As noted in the code, I wanted to have `ED_armature_pose_transform_mirror_update_relative` in `armature_utils.c`. However, that function depends on the TransData type which is only available in `editors/transform/transform.h`. I'm wondering if there is some nice way we could solve this issue without too may hacks.