Css mixins reused across Nri modules.
selectionShadow : List Css.Style
Draw a 2 px thick ochre border around the element to indicate it is selected.
This uses a CSS box shadow to draw what looks like a border. Box shadows are perfect for this because they don't affect the elements positioning in any way. This means we can be sure switching the selection shadow on and off is not going to make the element jump.