custom-default-slug.php

<?php
add_filter( 'get_sample_permalink_html', 'willy_filter_fake_permalink', 10, 4 );
function willy_filter_fake_permalink( $return, $id, $newtitle, $newslug ) {
    remove_filter( 'get_sample_permalink_html', 'willy_filter_fake_permalink' );
    if ( $newslug === NULL ) {
        if ( 'post' == get_post_type( $id ) ) {
            $return = preg_replace( "/id=\"editable-post-name\"([^>]*)>/", "id=\"editable-post-name\"$1>actualite-", $return );
            $return = preg_replace( "/id=\"editable-post-name-full\">/", "id=\"editable-post-name-full\">actualite-", $return );
        }
    }
    return $return;
}