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/SConnection.h b/common/rfb/SConnection.h
index 63dc314..bc43583 100644
--- a/common/rfb/SConnection.h
+++ b/common/rfb/SConnection.h
@@ -73,6 +73,7 @@
 
     virtual void setEncodings(int nEncodings, const rdr::S32* encodings);
 
+    virtual void supportsQEMUKeyEvent();
 
     // Methods to be overridden in a derived class