Update get_walked_pages to support custom post types Fixes 10up#200

When reordering a custom post type, this `get_walked_pages` call fails with an `undefined array key` error on line 433.

Digging into it more, we can see that the `get_walked_pages` call is outputting only the `top_level_pages` for the `page` post type.

This is due to the `get_pages` call on line 200 (https://github.com/10up/simple-page-ordering/blob/develop/class-simple-page-ordering.php#L200) defaulting to [`post_type => 'page'`](https://developer.wordpress.org/reference/functions/get_pages/#parameters).

If we tweak this to pass in the `post_type` of the post we're working with, in the same way we do with [`is_post_type_sortable`](https://github.com/10up/simple-page-ordering/blob/develop/class-simple-page-ordering.php#L244), this issue will be resolved.

class-simple-page-ordering.php

@@ -124,7 +124,7 @@ public static function handle_move_under_sibling( $post_id ) {
 				wp_die( esc_html__( 'You are not allowed to edit this item.', 'simple-page-ordering' ) );
 			}
 
-			list( 'top_level_pages' => $top_level_pages, 'children_pages' => $children_pages ) = self::get_walked_pages();
+			list( 'top_level_pages' => $top_level_pages, 'children_pages' => $children_pages ) = self::get_walked_pages( $post->post_type );
 
 			// Get the relevant siblings.
 			if ( 0 === $post->post_parent ) {
@@ -195,9 +195,9 @@ public static function redirect_to_referer() {
 		 *    @type WP_Post[] $children_pages  Children pages.
 		 * }
 		 */
-		public static function get_walked_pages() {
+		public static function get_walked_pages( $post_type = 'page' ) {
 			global $wpdb;
-			$pages = get_pages( array( 'sort_column' => 'menu_order title' ) );
+			$pages = get_pages( array( 'sort_column' => 'menu_order title', 'post_type' => $post_type ) );
 
 			$top_level_pages = array();
 			$children_pages  = array();
@@ -393,7 +393,7 @@ public static function page_row_actions( $actions, $post ) {
 				return $actions;
 			}
 
-			list( 'top_level_pages' => $top_level_pages, 'children_pages' => $children_pages ) = self::get_walked_pages();
+			list( 'top_level_pages' => $top_level_pages, 'children_pages' => $children_pages ) = self::get_walked_pages( $post->post_type );
 
 			$edit_link                   = get_edit_post_link( $post->ID, 'raw' );
 			$move_under_grandparent_link = add_query_arg(