Fixing positioning of mask for stacking modals.

If you have stacking modals, the mask does not reposition itself to be behind the widget on top.

The problem seems to stem from: [Widget Modality](http://yuilibrary.com/yui/docs/api/files/widget-modality_js_Widget-Modality.js.html#l530)

If both widgets are modal, it finds the container and inserts the mask BEFORE the FIRST child, but should really be before the current node `bb`, or the mask will never exist between widgets.