kses needs to strip and add slashes to posts

[donncha]

kses needs to strip and add slashes to posts:
ie:

function wp_filter_post_kses($data) {
        global $allowedposttags;
        return addslashes( wp_kses(stripslashes( $data ), $allowedposttags) );
}

[ryan]

(In [2902]) stripslashes before running kses. Props donncha. fixes #1697

[hakre]

Comment after the impression of #4723: hmm, if someone could acutally say why?