AudacityApp emits an event when the global clipboard changes

src/AudacityApp.h

@@ -50,6 +50,11 @@ void QuitAudacity();
 
 extern bool gIsQuitting;
 
+// An event emitted by the application whenever the global clipboard's
+// contents change.
+wxDECLARE_EXPORTED_EVENT( AUDACITY_DLL_API,
+                          EVT_CLIPBOARD_CHANGE, wxCommandEvent);
+
 // Asynchronous open
 DECLARE_EXPORTED_EVENT_TYPE(AUDACITY_DLL_API, EVT_OPEN_AUDIO_FILE, -1);