Creates managed widget for WidgetView.

The WidgetView class originally could have several widgets. Now it comes with one managed widget and will be instantiated automatically. Child widgets should be added to the managed widget directly.

The managed widget should never set bounds directly. Instead, set the bounds for its WidgetView and the managed widget will have the same size matched to the WidgetView.

Signed-off-by: Olli Wang <olliwang@ollix.com>