add flags to permissions groups

Flags can be added to permissions group to store some metadata.

An interface `PermissionsGroupFlagProvider` allow to add some flag to the form which create/
edit permissions groups.

Entity/GroupCenter.php

@@ -58,7 +58,7 @@ class GroupCenter
     
     public function __construct()
     {
-        $this->permissionGroups = new ArrayCollection();
+        $this->permissionsGroup = new ArrayCollection();
         $this->users = new ArrayCollection();
     }