Blender Git Loki

Multi-Object Pose: POSE_OT_group_assign/unassign

Like the select/deselect operators, we only support doing this for the
active object for now. From the UI panels, it doesn't make sense to be
operating on bone groups which may not appear/line up with those in the panel
that we're currently dealing with.

This can be reviewed later, but for now, it's easier and makes more sense
this way.