Clean up internal clipboard handling

We now filter incoming data, which means we can start assuming the
clipboard data is always null terminated. This allows us to clean
up a lot of the internal handling.
diff --git a/win/rfb_win32/Clipboard.h b/win/rfb_win32/Clipboard.h
index 3da7bfc..c69e981 100644
--- a/win/rfb_win32/Clipboard.h
+++ b/win/rfb_win32/Clipboard.h
@@ -1,4 +1,5 @@
 /* Copyright (C) 2002-2005 RealVNC Ltd.  All Rights Reserved.
+ * Copyright 2016-2019 Pierre Ossman for Cendio AB
  * 
  * This is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -38,7 +39,7 @@
       // -=- Abstract base class for callback recipients
       class Notifier {
       public:
-        virtual void notifyClipboardChanged(const char* text, int len) = 0;
+        virtual void notifyClipboardChanged(const char* text) = 0;
         virtual ~Notifier() {};
       };