Make wxStandardPaths::AppendAppInfo() public

This function can be useful to the application code and will be used by
wxFileConfig in the upcoming commits.

interface/wx/stdpaths.h

@@ -195,6 +195,20 @@ public:
         ConfigFileConv_Ext
     };
 
+    /**
+        Append application and/or vendor name to the given directory.
+
+        By default, appends the subdirectory with the application name, as
+        returned by wxApp::GetAppName(), to the given directory.
+
+        This behaviour is affected by UseAppInfo(), e.g. if this function is
+        called with `AppInfo_VendorName` then the vendor name would be appended
+        instead of the application name.
+
+        @since 3.3.0
+    */
+    wxString AppendAppInfo(const wxString& dir) const;
+
     /**
         MSW-specific function undoing the effect of IgnoreAppSubDir() calls.
 
@@ -513,6 +527,8 @@ public:
 
         By default, only the application name is used.
 
+        @see AppendAppInfo()
+
         @since 2.9.0
     */
     void UseAppInfo(int info);