Add wxStandardPaths::ConfigFileConv enum for clarity and safety

Use an enum instead of type-unsafe "int" for the second parameter of the
recently added wxStandardPaths::MakeConfigFileName().

This also avoids unnatural dependency of wxStandardPaths on
wxCONFIG_USE_SUBDIR constant defined in a higher level wxFileConfig class.

No real changes, but just make things a bit more robust and hopefully more
clear.

src/msw/stdpaths.cpp

@@ -370,7 +370,9 @@ wxString wxStandardPaths::GetPluginsDir() const
 }
 
 
-wxString wxStandardPaths::MakeConfigFileName(const wxString& basename, int WXUNUSED(style)) const
+wxString
+wxStandardPaths::MakeConfigFileName(const wxString& basename,
+                                    ConfigFileConv WXUNUSED(conv)) const
 {
     wxFileName fn(wxEmptyString, basename);
     fn.SetExt(wxT("ini"));