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/vncviewer/DesktopWindow.h b/vncviewer/DesktopWindow.h
index fe938d9..6b03325 100644
--- a/vncviewer/DesktopWindow.h
+++ b/vncviewer/DesktopWindow.h
@@ -62,9 +62,6 @@
   // Resize the current framebuffer, but retain the contents
   void resizeFramebuffer(int new_w, int new_h);
 
-  // Incoming clipboard from server
-  void serverCutText(const char* str);
-
   // New image for the locally rendered cursor
   void setCursor(int width, int height, const rfb::Point& hotspot,
                  const rdr::U8* data);
@@ -72,6 +69,11 @@
   // Change client LED state
   void setLEDState(unsigned int state);
 
+  // Clipboard events
+  void handleClipboardRequest();
+  void handleClipboardAnnounce(bool available);
+  void handleClipboardData(const char* data);
+
   // Fl_Window callback methods
   void draw();
   void resize(int x, int y, int w, int h);