Each .cpp/.mm file includes corresponding header before any other...

... except Audacity.h

This forces us to make each header contain all forward declarations or nested
headers that it requires, rather than depend on context.

src/widgets/ErrorDialog.cpp

@@ -20,6 +20,7 @@
 *//********************************************************************/
 
 #include "../Audacity.h"
+#include "ErrorDialog.h"
 
 #include <wx/button.h>
 #include <wx/icon.h>
@@ -42,8 +43,6 @@
 #include "../Prefs.h"
 #include "HelpSystem.h"
 
-#include "ErrorDialog.h"
-
 // special case for alias missing dialog because we keep track of if it exists.
 class AliasedFileMissingDialog final : public ErrorDialog
 {