[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.

modules/Favorite/lib/favcommand.php

@@ -0,0 +1,27 @@
+<?php
+
+class FavCommand extends Command 
+{ 
+    var $other = null; 
+ 
+    function __construct($user, $other) 
+    { 
+        parent::__construct($user); 
+        $this->other = $other; 
+    } 
+ 
+    function handle($channel) 
+    { 
+        $notice = $this->getNotice($this->other); 
+ 
+        try {
+            $fave = Fave::addNew($this->user->getProfile(), $notice); 
+        } catch (Exception $e) {
+            $channel->error($this->user, $e->getMessage());
+            return;
+        }
+ 
+        // TRANS: Text shown when a notice has been marked as favourite successfully. 
+        $channel->output($this->user, _('Notice marked as fave.')); 
+    } 
+}