MDL-67115 core: php74 params order for implode()/join()

Passing parameters to implode() in reverse order is deprecated, use
implode($glue, $parts) instead of implode($parts, $glue).

This commit corresponds to phpunit and manual detections, core files.

lib/listlib.php

@@ -552,7 +552,7 @@ abstract class list_item {
         } else {
             $childrenhtml = '';
         }
-        return $this->item_html($extraargs).' '.(join($this->icons, '')).(($childrenhtml !='')?("\n".$childrenhtml):'');
+        return $this->item_html($extraargs).' '.(join('', $this->icons)).(($childrenhtml !='')?("\n".$childrenhtml):'');
     }
 
     public function set_icon_html($first, $last, $lastitem) {