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.cxx b/vncviewer/DesktopWindow.cxx
index 150c39b..4429e77 100644
--- a/vncviewer/DesktopWindow.cxx
+++ b/vncviewer/DesktopWindow.cxx
@@ -276,9 +276,9 @@
 }
 
 
-void DesktopWindow::serverCutText(const char* str, rdr::U32 len)
+void DesktopWindow::serverCutText(const char* str)
 {
-  viewport->serverCutText(str, len);
+  viewport->serverCutText(str);
 }