Move server cut text handler to Viewport

That way both incoming and outgoing clipboard are both in the same
place, making things clearer.
diff --git a/vncviewer/DesktopWindow.cxx b/vncviewer/DesktopWindow.cxx
index 306803f..c450ea9 100644
--- a/vncviewer/DesktopWindow.cxx
+++ b/vncviewer/DesktopWindow.cxx
@@ -276,6 +276,12 @@
 }
 
 
+void DesktopWindow::serverCutText(const char* str, rdr::U32 len)
+{
+  viewport->serverCutText(str, len);
+}
+
+
 void DesktopWindow::setCursor(int width, int height,
                               const rfb::Point& hotspot,
                               const rdr::U8* data)