screen.php tidying. Doc fixes and improvements. Improve logic in...

screen.php tidying. Doc fixes and improvements. Improve logic in ::show_screen_options(). Remove regex in ::set_parentage(). Better variable names. Streamline get_screen_icon(). Whitespace. see #18785.

git-svn-id: https://core.svn.wordpress.org/trunk@19051 1a063a9b-81f0-0310-95a4-ce76da25c4cd