fix add_widget to use correct size_y when adding rows

The call to this.add_faux_rows on 252 used to get passed null whenever col/row were defined for a new widget. This led to only a single row being added to the faux grid and insufficient css rules being generated in generate_stylesheet. 

http://jsfiddle.net/uHcHq/

Try dragging the 1 box on top of the 0 and then the 2 box on top of either the 0 or 1.