Logged out warnings:
- Replace the Close button with an always visible "X" icon in the top/right corner.
- Check if the user is still logged in every 3 min. by default.
- Add 'wp_auth_check_interval' filter so the interval can be set from PHP.
See #23295.

git-svn-id: https://core.svn.wordpress.org/trunk@24695 1a063a9b-81f0-0310-95a4-ce76da25c4cd