[
  {
    "id" : "layout",
    "type" : "ClutterBoxLayout",
    "vertical" : true,
    "spacing" : 12,
    "pack-start" : false
  },
  {
    "id" : "test",
    "type" : "ClutterBox",
    "layout-manager" : "layout", 
    "children" : [
      {
        "id" : "child-1",
        "type" : "ClutterRectangle",
        "width" : "3 em",
        "height" : "3 em"
      }
    ]
  }
]