Add xhandler hook

Boiler plate code to intercept system events from FLTK so that
we can generate proper keyboard messages.
diff --git a/vncviewer/Viewport.h b/vncviewer/Viewport.h
index d9eea35..30dacad 100644
--- a/vncviewer/Viewport.h
+++ b/vncviewer/Viewport.h
@@ -73,6 +73,8 @@
   void handleKeyPress(int keyCode, rdr::U32 keySym);
   void handleKeyRelease(int keyCode);
 
+  static bool handleXEvent(void *event, void *data);
+
   rdr::U32 translateKeyEvent(void);
   void handleFLTKKeyPress(void);