Licensing changes:

1. add the option the code to be used under GPLv2 or GPLv3; previously only GPLv2 was allowed

2. add the "proxy option" as described in GPLv3 so we can add future licenses without having to bulk-edit everything again :)

3. remove the awkward "exception for MIT code in Mac client" clause; it was unnecessary and confusing.

gtk/open-dialog.h

@@ -1,11 +1,8 @@
 /*
  * This file Copyright (C) 2010-2014 Mnemosyne LLC
  *
- * This file is licensed by the GPL version 2. Works owned by the
- * Transmission project are granted a special exemption to clause 2 (b)
- * so that the bulk of its code can remain under the MIT license.
- * This exemption does not extend to derived works not owned by
- * the Transmission project.
+ * It may be used under the GNU Public License v2 or v3 licenses,
+ * or any future license endorsed by Mnemosyne LLC.
  *
  * $Id$
  */