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.
Built from https://develop.svn.wordpress.org/trunk@27702


git-svn-id: http://core.svn.wordpress.org/trunk@27541 1a063a9b-81f0-0310-95a4-ce76da25c4cd