Add WP_Screen methods get_help_tabs(), get_help_tab( $id ),...

Add WP_Screen methods get_help_tabs(), get_help_tab( $id ), get_help_sidebar(). Store help tabs by tab ID, not numeric key; allows proper removal with remove_help_tab( $id ). props BenChapman, npetetin for initial getter patch. fixes #19974.

git-svn-id: https://core.svn.wordpress.org/trunk@19944 1a063a9b-81f0-0310-95a4-ce76da25c4cd