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.

interface/wx/stdpaths.h

@@ -79,6 +79,18 @@ public:
         */
         Dir_Cache,
 
+        /**
+            Directory containing configuration information.
+
+            Example return values:
+            - Unix: `~/.config`
+            - Windows: `C:\Users\username\AppData\Roaming`
+            - Mac: @c `~/Library/Preferences`
+
+            @since 3.3.0
+        */
+        Dir_Config,
+
         /**
             Directory containing user documents.
 
@@ -384,8 +396,8 @@ public:
         - Mac: @c ~/Library/Preferences
 
         Only use this method if you have a single configuration file to put in this
-        directory, otherwise GetUserDataDir() is more appropriate as the latter
-        adds @c appinfo to the path, unlike this function.
+        directory, otherwise calling AppendAppInfo() with the value returned by
+        GetDir() with wxStandardPaths::Dir_Config is more appropriate.
     */
     virtual wxString GetUserConfigDir() const;