FR#23 – Add a "toggle statusbar" key binding

One of the reasons for why I like calcurse so much is the clean and minimal
interface, but I think there is one thing that takes away a bit from it, and
that is the hotkey legend at the bottom. I would really like to see a key for
toggling it, or have a config option to hide it. It's still really useful so I
think a toggle would be the best in case you forget some key.