The 
gtk-action-bar widget is designed to present contextual actions.  
  It is expected to be displayed below the content and expand horizontally to
  fill the area.  
  It allows placing children at the start or the end. In addition, it contains
  an internal centered box which is centered with respect to the full width of
  the box, even if the children at either side take up different amounts of
  space.  
CSS nodes
    The 
gtk-action-bar implementation has a single CSS node with name    
actionbar.    
Child Property Details
          - pack-type
 -         The pack-type child property of type gtk-pack-type        (Read / Write) 
        A value of the gtk-pack-type enumeration indicating whether the
        child widget is packed with reference to the start or end of the parent.        
        Default value: :start             - position
 -         The position child property of type :int (Read / Write)        
        The index of the child widget in the parent. 
        Allowed values: >= -1 
        Default value: 0