Introduce a new generic CSS clearfix utility class.

`.wp-clearfix` is now the recommended way to clear and contain floated elements.
Adds back compatibility for the `.nav-tab-wrapper` navigation tabs.

See #26396.
Built from https://develop.svn.wordpress.org/trunk@36171


git-svn-id: http://core.svn.wordpress.org/trunk@36138 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/site-themes.php

@@ -149,7 +149,7 @@ require( ABSPATH . 'wp-admin/admin-header.php' ); ?>
 <div class="wrap">
 <h1 id="edit-site"><?php echo $title; ?></h1>
 <p class="edit-site-actions"><a href="<?php echo esc_url( get_home_url( $id, '/' ) ); ?>"><?php _e( 'Visit' ); ?></a> | <a href="<?php echo esc_url( get_admin_url( $id ) ); ?>"><?php _e( 'Dashboard' ); ?></a></p>
-<h2 class="nav-tab-wrapper nav-tab-small">
+<h2 class="nav-tab-wrapper nav-tab-small wp-clearfix">
 <?php
 $tabs = array(
 	'site-info'     => array( 'label' => __( 'Info' ),     'url' => 'site-info.php'     ),