Add support for cache directory to wxStandardPaths::GetUserDir()

This has direct equivalent under macOS and when using XDG.

See #17727.

interface/wx/stdpaths.h

@@ -69,6 +69,16 @@ public:
     /// Possible values for userDir parameter of GetUserDir().
     enum Dir
     {
+        /**
+            Directory for caching files.
+
+            Example return values:
+            - Unix: @c ~/.cache
+            - Windows: @c "C:\Users\username\AppData\Local"
+            - Mac: @c ~/Library/Caches
+        */
+        Dir_Cache,
+
         /**
             Directory containing user documents.