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.

include/wx/unix/stdpaths.h

@@ -49,7 +49,8 @@ public:
 #ifndef __VMS
     virtual wxString GetUserDir(Dir userDir) const wxOVERRIDE;
 #endif
-    virtual wxString MakeConfigFileName(const wxString& basename, int style) const wxOVERRIDE;
+    virtual wxString MakeConfigFileName(const wxString& basename,
+                                        ConfigFileConv conv) const wxOVERRIDE;
 
 protected:
     // Ctor is protected, use wxStandardPaths::Get() instead of instantiating