(Re-)adds display: none; to Thickbox content to insert a download.

The Thickbox content to help you insert a downloads shortcode was shown because the `display: none` was removed from the style attribute in the `release/1.9` branch

![add_new_form__wp_latest__wordpress_20131223_221708](https://f.cloud.github.com/assets/885856/1803272/9c7e16ac-6c17-11e3-942f-5cde213df9c4.jpg)

The contents of the Thickbox is still somewhat messed up by the way.

![add_new_form__wp_latest__wordpress_20131223_221917](https://f.cloud.github.com/assets/885856/1803285/e870f836-6c17-11e3-919e-48a7a9172354.jpg)