Automatically whitelist screen options added via add_screen_option()

[mdawaffe]

set_screen_options() currently maintains a hardcoded whitelist of screens that can set options.

The list is extendable via the filter:

apply_filters('set-screen-option', false, $option, $value);

but that's sort of a pain. If a plugin calls add_screen_option(), it should be added to the whitelist automatically.

Also note that set_screen_options() is called so early, there's no convenient place for plugins to add to the set-screen-option filter. The best workaround I've been able to come up with is to manually call set_screen_options() again inside the page's load-$hook.

Example naive plugin code that does not work in <=3.3.

# Inside a menu page's load-$hook
// Display per-page settings in screen options
add_screen_option( 'per_page', array( 'label' => __( 'Things' ) ) );

We may be able to get away with killing this whitelist come 3.4.