Update meta box functions to handle WP_Screen objects and pass objects instead...

Update meta box functions to handle WP_Screen objects and pass objects instead of IDs to them in core files.  Allow passing emptiness to get the current screen.  see #18958

git-svn-id: https://core.svn.wordpress.org/trunk@19013 1a063a9b-81f0-0310-95a4-ce76da25c4cd