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/DesktopWindow.h b/vncviewer/DesktopWindow.h
index 6ec8e1b..fe938d9 100644
--- a/vncviewer/DesktopWindow.h
+++ b/vncviewer/DesktopWindow.h
@@ -63,7 +63,7 @@
   void resizeFramebuffer(int new_w, int new_h);
 
   // Incoming clipboard from server
-  void serverCutText(const char* str, rdr::U32 len);
+  void serverCutText(const char* str);
 
   // New image for the locally rendered cursor
   void setCursor(int width, int height, const rfb::Point& hotspot,