Basic support for QEMU Extended Key Events

This adds the basic infrastructure and handshake for the QEMU
Extended Key Events extension. No viewer or server makes use of
the extra functionality yet though.
diff --git a/common/rfb/encodings.h b/common/rfb/encodings.h
index adeecaa..122afe7 100644
--- a/common/rfb/encodings.h
+++ b/common/rfb/encodings.h
@@ -40,6 +40,7 @@
   const int pseudoEncodingFence = -312;
   const int pseudoEncodingContinuousUpdates = -313;
   const int pseudoEncodingCursorWithAlpha = -314;
+  const int pseudoEncodingQEMUKeyEvent = -258;
 
   // TightVNC-specific
   const int pseudoEncodingLastRect = -224;