Switch to using display:none rather that moving the box.

A code improvement [suggested by Peter](https://github.com/adobe/brackets/pull/9292#issuecomment-56929459)
@dangoor Out of curiosity (_not_ because I think we need to change this a 3rd time :-) ... could the left-hand clause be `undefined` or `null` or `""` to tell React to leave the `display` CSS property unchanged at its default value?  (That way we wouldn't have to hardcode whether it's `block` vs. `inline-block`, etc.)