Add wxStandardPaths::Dir_Config

This allows using GetUserDir(Dir_Config) to retrieve XDG-compliant
location of the configuration files, independently of the file layout
used by the global wxStandardPaths object.

Recommend using this directory instead of GetUserDataDir() for multiple
configuration files under Unix.

include/wx/stdpaths.h

@@ -51,6 +51,7 @@ public:
     enum Dir
     {
         Dir_Cache,
+        Dir_Config,
         Dir_Documents,
         Dir_Desktop,
         Dir_Downloads,