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/HelpText.h

@@ -11,6 +11,8 @@
 #ifndef __AUDACITY_HELP_TEXT__
 #define __AUDACITY_HELP_TEXT__
 
+class wxString;
+
 wxString HelpText( const wxString & Key );
 wxString TitleText( const wxString & Key );