Improved clipboard API

Change the internal clipboard API to use a request based model in
order to be prepared for more advanced clipboard transfers.
diff --git a/win/rfb_win32/Clipboard.h b/win/rfb_win32/Clipboard.h
index c69e981..1dead82 100644
--- a/win/rfb_win32/Clipboard.h
+++ b/win/rfb_win32/Clipboard.h
@@ -39,7 +39,7 @@
       // -=- Abstract base class for callback recipients
       class Notifier {
       public:
-        virtual void notifyClipboardChanged(const char* text) = 0;
+        virtual void notifyClipboardChanged(bool available) = 0;
         virtual ~Notifier() {};
       };
 
@@ -49,6 +49,9 @@
       // - Set the notifier to use
       void setNotifier(Notifier* cbn) {notifier = cbn;}
 
+      // - Get the clipboard contents
+      char* getClipText();
+
       // - Set the clipboard contents
       void setClipText(const char* text);