Widget Customizer: Improve widget highlighting.

* Prefix box-shadow in inject_preview_css()
* Remove border-radius in inject_preview_css()
* Prevent hidden box-shadows in Customizer

see #27358.

git-svn-id: https://develop.svn.wordpress.org/trunk@27702 602fd350-edb4-49c9-b593-d223f7449a82