'UI_context_copy_to_selected_list()' was working fine for constraints on
objects (would properly get the path and list of selected objects) and
also for bone settings (would properly get the path and list of bone).
There was no special case for constraints on bones though, so code would
get the path right, but only get a list of selected objects (not
selected bones).

So we now get the right bones and resolve the path to the constraint
setting _from the bones_.