Move socket write event handling in to the RFB core

What to do when a socket is writeable should be handled in the
RFB core code as there may be other events we want to fire off
when this happens.
diff --git a/common/rfb/VNCSConnectionST.h b/common/rfb/VNCSConnectionST.h
index 72ffc1d..55b7ca3 100644
--- a/common/rfb/VNCSConnectionST.h
+++ b/common/rfb/VNCSConnectionST.h
@@ -65,6 +65,9 @@
     // Socket if an error occurs, via the close() call.
     void processMessages();
 
+    // flushSocket() pushes any unwritten data on to the network.
+    void flushSocket();
+
     // Called when the underlying pixelbuffer is resized or replaced.
     void pixelBufferChange();