Use XDG-compliant location by default in wxFileConfig

Still use the traditional dot file if it already exists, but prefer
using the new location otherwise, i.e. for the new program installation.

Add wxCONFIG_USE_HOME to allow forcing the use of the old location if
really necessary.

Also use the new style as default "old style" of MigrateLocalFile() so
that calling it even when using XDG layout in wxStandardPaths still
works as expected.

include/wx/fileconf.h

@@ -137,7 +137,9 @@ public:
       wxString error;
   };
   static MigrationResult
-  MigrateLocalFile(const wxString& name, int newStyle, int oldStyle = 0);
+  MigrateLocalFile(const wxString& name,
+                   int newStyle,
+                   int oldStyle = wxCONFIG_USE_HOME);
 
   // ctor & dtor
     // New constructor: one size fits all. Specify wxCONFIG_USE_LOCAL_FILE or