Improve keyboard shortcut discoverabilty

[vitalybe]

I see 2 possible improvements:

1. The keybinding screen should show the keybindings of the current view, then all the common keyboard bindings. It should hide all the key bindings that can't be used in the current view.

2. Show the most useful shortcuts in the bottom, similar to the way it is shown in Midnight Commander:

[pzrq]

This would be nice. I didn't know the Manual existed and had a Default Keybindings section until a moment ago.