Add an option to map Menu to Win

The menu key is rarely used these days, so I tend to bind it to something more common. There are already options to use it as Compose or for changing layouts, but none for binding it to Super, which I like because the key is in a nicely symmetrical position to the Windows key on most keyboards. (Note that there's altwin(alt_super_win), but it changes quite a few other things as well...)