Debugger: Add work notification hook to UserInterface.

UserInterface:
- Add new hook function used to notify the UI that some form of
  background work is taking place for informational purposes,
  i.e. no interaction required. Implement accordingly in
  GraphicalUserInterface.

src/apps/debugger/user_interface/cli/CommandLineUserInterface.cpp

@@ -203,6 +203,12 @@ CommandLineUserInterface::NotifyUser(const char* title, const char* message,
 }
 
 
+void
+CommandLineUserInterface::NotifyBackgroundWorkStatus(const char* message)
+{
+}
+
+
 int32
 CommandLineUserInterface::SynchronouslyAskUser(const char* title,
 	const char* message, const char* choice1, const char* choice2,