[CORE] Move core plugins to a new modules directory

For reference (raised by rozzin in IRC):

* http://foldoc.org/module
* http://foldoc.org/library
* http://foldoc.org/plugin

As noted by XRevan86, modules are not necessarily non-essential.
As we will keep the modules directory in GS root [therefore, near to
plugins/], it is evidenced the difference between both.

This is a simple yet fundamental structural change. It doesn't change
functionality but makes clearer the way we understand GNU social's
internals.

lib/event.php

@@ -82,7 +82,7 @@ class Event {
      *                 on results of handlers.
      */
 
-    public static function handle($name, array $args=array()) {
+    public static function handle($name, array $args = []) {
         $result = null;
         if (array_key_exists($name, Event::$_handlers)) {
             foreach (Event::$_handlers[$name] as $handler) {