add featured image thumbnail to WordPress admin columns

Add featured image thumbnail to admin columns list for posts and pages:

<?php
// add featured image thumbnails to WordPress admin columns
// add_theme_support( 'post-thumbnails' ); // theme should support
function themename_add_post_thumbnail_column( $cols ) { // add the thumb column
	// output feature thumb in the end
	//$cols['themename_post_thumb'] = __( 'Featured image', 'themename' );
	//return $cols;
	// output feature thumb in a different column position
	$cols_start = array_slice( $cols, 0, 2, true );
	$cols_end   = array_slice( $cols, 2, null, true );
	$custom_cols = array_merge(
		$cols_start,
		array( 'themename_post_thumb' => __( 'Featured image', 'themename' ) ),
		$cols_end
	);
	return $custom_cols;
}
add_filter( 'manage_posts_columns', 'themename_add_post_thumbnail_column', 5 ); // add the thumb column to posts
add_filter( 'manage_pages_columns', 'themename_add_post_thumbnail_column', 5 ); // add the thumb column to pages

function themename_display_post_thumbnail_column( $col, $id ) { // output featured image thumbnail
	switch( $col ){
		case 'themename_post_thumb':
			if( function_exists( 'the_post_thumbnail' ) ) {
				echo the_post_thumbnail( 'thumbnail' );
			} else {
				echo __( 'Not supported in theme', 'themename' );
			}
			break;
	}
}
add_action( 'manage_posts_custom_column', 'themename_display_post_thumbnail_column', 5, 2 ); // add the thumb to posts
add_action( 'manage_pages_custom_column', 'themename_display_post_thumbnail_column', 5, 2 ); // add the thumb to pages
?>