Debugger: Cleanup.

- Split MessageCodes.h into a second file that separates out the
  application-specific message codes from those used by the core.
  Adjust includes accordingly. No functional change.

src/apps/debugger/user_interface/gui/team_window/SourceView.cpp

@@ -31,6 +31,7 @@
 #include <AutoLocker.h>
 #include <ObjectList.h>
 
+#include "AppMessageCodes.h"
 #include "AutoDeleter.h"
 #include "Breakpoint.h"
 #include "DisassembledCode.h"