Store screen help and options as static data against WP_Screen. Individual...

Store screen help and options as static data against WP_Screen. Individual screen objects no longer hold data it can't re-generate on construction or otherwise fetch. convert_to_screen() now returns a WP_Screen object. Various globals are gone. Introduces WP_Screen::get_option(). Allows for a formal factory to be introduced later. see #18785.

git-svn-id: https://core.svn.wordpress.org/trunk@18943 1a063a9b-81f0-0310-95a4-ce76da25c4cd