Move AudacityMessageBox to its own files...

... breaking cycles among low-level files introduced by 273ba9f

src/widgets/AudacityMessageBox.cpp

@@ -0,0 +1,17 @@
+/**********************************************************************
+
+  Audacity: A Digital Audio Editor
+
+  AudacityMessageBox.cpp
+
+  Paul Licameli split this out of ErrorDialog.cpp
+
+**********************************************************************/
+
+#include "AudacityMessageBox.h"
+#include "../Internat.h"
+
+wxString AudacityMessageBoxCaptionStr()
+{
+   return _("Message");
+}