fix(mail): improve decoding of ms-tnef body parts

- added decoding of meeting request & reponse
- added SOGoTnefDecoderDebugEnabled system default

SoObjects/SOGo/SOGoSystemDefaults.m

@@ -1,6 +1,6 @@
 /* SOGoSystemDefaults.m - this file is part of SOGo
  *
- * Copyright (C) 2009-2015 Inverse inc.
+ * Copyright (C) 2009-2021 Inverse inc.
  * Copyright (C) 2012 Jeroen Dekkers <jeroen@dekkers.ch>
  *
  * This file is free software; you can redistribute it and/or modify
@@ -434,6 +434,11 @@ _injectConfigurationFromFile (NSMutableDictionary *defaultsDict,
   return [self boolForKey: @"SOGoEASDebugEnabled"];
 }
 
+- (BOOL) tnefDecoderDebugEnabled
+{
+  return [self boolForKey: @"SOGoTnefDecoderDebugEnabled"];
+}
+
 - (BOOL) xsrfValidationEnabled
 {
   id o;