Kills setup_output definitively, saving some indirection. set_scan_target takes its place.

Outputs/ScanTarget.cpp

@@ -0,0 +1,13 @@
+//
+//  ScanTarget.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/11/2018.
+//  Copyright © 2018 Thomas Harte. All rights reserved.
+//
+
+#include "ScanTarget.hpp"
+
+using namespace Outputs::Display;
+
+NullScanTarget NullScanTarget::singleton;