Rework get_hidden_meta_boxes() to leverage a full WP_Screen object. Prevents...

Rework get_hidden_meta_boxes() to leverage a full WP_Screen object. Prevents custom post types from having their explicity supported meta boxes being hidden by default. see #18958.

git-svn-id: https://core.svn.wordpress.org/trunk@19049 1a063a9b-81f0-0310-95a4-ce76da25c4cd