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/vncviewer/CConn.h b/vncviewer/CConn.h
index 2e3362c..38e09c2 100644
--- a/vncviewer/CConn.h
+++ b/vncviewer/CConn.h
@@ -61,7 +61,7 @@
 
   void bell();
 
-  void serverCutText(const char* str, rdr::U32 len);
+  void serverCutText(const char* str);
 
   void framebufferUpdateStart();
   void framebufferUpdateEnd();