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/msw/stdpaths.h

@@ -26,7 +26,7 @@ public:
     virtual wxString GetUserDataDir() const;
     virtual wxString GetUserLocalDataDir() const;
     virtual wxString GetPluginsDir() const;
-    virtual wxString GetDocumentsDir() const;
+    virtual wxString GetUserDir(Dir userDir) const wxOVERRIDE;
 
 
     // MSW-specific methods
@@ -72,6 +72,8 @@ protected:
     // get the path corresponding to the given standard CSIDL_XXX constant
     static wxString DoGetDirectory(int csidl);
 
+    static wxString DoGetKnownFolder(const GUID& rfid);
+
     // return the directory of the application itself
     wxString GetAppDir() const;