General: Introduce a wp_list_sort() helper function, v2.

In addition to `wp_list_filter()` for filtering a list of objects, and `wp_list_pluck()` for plucking a certain field out of each object in a list, this new function can be used for sorting a list of objects by specific fields. These functions are now all contained within the new `WP_List_Util()` class and `wp_list_sort()` is used in various parts of core for sorting lists.

This was previously committed in [38859] but got reverted in [38862] and [38863]. To fix the previous issues, `wp_list_sort()` supports now an additional argument to preserve array keys via `uasort()`.

Props flixos90, DrewAPicture, jorbin.
Fixes #37128.

git-svn-id: https://develop.svn.wordpress.org/trunk@38928 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/category-template.php

@@ -99,45 +99,6 @@ function get_the_category( $id = false ) {
 	return apply_filters( 'get_the_categories', $categories, $id );
 }
 
-/**
- * Sort categories by name.
- *
- * Used by usort() as a callback, should not be used directly. Can actually be
- * used to sort any term object.
- *
- * @since 2.3.0
- * @access private
- *
- * @param object $a
- * @param object $b
- * @return int
- */
-function _usort_terms_by_name( $a, $b ) {
-	return strcmp( $a->name, $b->name );
-}
-
-/**
- * Sort categories by ID.
- *
- * Used by usort() as a callback, should not be used directly. Can actually be
- * used to sort any term object.
- *
- * @since 2.3.0
- * @access private
- *
- * @param object $a
- * @param object $b
- * @return int
- */
-function _usort_terms_by_ID( $a, $b ) {
-	if ( $a->term_id > $b->term_id )
-		return 1;
-	elseif ( $a->term_id < $b->term_id )
-		return -1;
-	else
-		return 0;
-}
-
 /**
  * Retrieve category name based on category ID.
  *