Support extended clipboard transfers

Implements support in both client and server for the extended
clipboard format first seen in UltraVNC. Currently only implements
text handling, but that is still an improvement as it extends the
clipboard from ISO 8859-1 to full Unicode.
diff --git a/common/rfb/encodings.h b/common/rfb/encodings.h
index acb86ec..cf0c857 100644
--- a/common/rfb/encodings.h
+++ b/common/rfb/encodings.h
@@ -63,6 +63,9 @@
   const int pseudoEncodingVMwareCursor = 0x574d5664;
   const int pseudoEncodingVMwareLEDState = 0x574d5668;
 
+  // UltraVNC-specific
+  const int pseudoEncodingExtendedClipboard = 0xC0A1E5CE;
+
   int encodingNum(const char* name);
   const char* encodingName(int num);
 }