Make sure addChild can add raw DOMNode and that getChildren is consistent

Following this all usage of getChildren should be reviewed to see if they might break. Some might have to be changed to use findWidgets().

I have checked ContentPane as @wkeese suggested but ContentPane is checking if the resize() method is available on the children before calling it so I'm not sure if we must really change the code there to use findWidgets() as it won' t break.