Add wxStandardPaths::GetUserDir() to get Desktop, Download etc

All major supported platforms have well defined per-user directories to store
Downloads, Music, Pictures, Videos and the Desktop files. The new method
wxStandardPaths::GetUserDir() allows for a unified way to access these on MSW,
OS X and Unix (if XDG user dirs specification is implemented for the latter).

See https://github.com/wxWidgets/wxWidgets/pull/89

include/wx/osx/core/stdpaths.h

@@ -52,7 +52,7 @@ public:
     virtual wxString
     GetLocalizedResourcesDir(const wxString& lang,
                              ResourceCat category = ResourceCat_None) const;
-    virtual wxString GetDocumentsDir() const;
+    virtual wxString GetUserDir(Dir userDir) const wxOVERRIDE;
 
 protected:
     // Ctor is protected, use wxStandardPaths::Get() instead of instantiating