Add function to display current keyboard shortcuts

[rkratky]

It would be nice to have the ability to quickly view current keyboard-shortcut config. Something like ? in mutt, for example.

[cfangmeier]

This would actually be a bit tricky to implement, I think. One way to go about it would be to use an urwid.Overlay. However, this would have to be inserted as a new root in the widget tree with the current app as a child and the shortcut display as another child. Haven't fully thought through how much code would need to be touched as a result of this.

That said, I think this would be a nice feature.