js动态更换cascades中listview的layout

http://supportforums.blackberry.com/t5/Cascades-Development/how-to-change-listview-s-layout-by-javascript/td-p/2474233

要点：

page的attachedobjects中写layout对象，只有这样js才可以通过其id访问到。

attachedObjects: [        /*StackListLayout {            id:layout            orientation: LayoutOrientation.LeftToRight        }*/        GridListLayout {            id: layout            columnCount: 2        }    ]

qml直接访问对象不需要attachedobjects里面声明，但js则需要。