Rename 'border' attribute on ref elements to 'standalone'

[madsrasmussen]

We currently have a border attribute on a ref element which adds a border around the element. The attribute is supposed to be used when the element is in "standalone" mode - Only 1 item which is not part of a ref list.

We should rename the property to the purpose instead of the visual outcome so we can change the visuals later if we want to or combine them with extra padding etc.