NbtkButton: Fix incorrect use of clutter_container_add_actor
NbtkButton: Fix incorrect use of clutter_container_add_actor

bg_image is not a container

clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx