bug#50067: Context menus

[Juri Linkov]

>> Other programs don't show the keys in context menus.
>> The Human Interface Guidelines say:
>>
>>   Show keyboard shortcuts in menu bar menus, not contextual menus.
>>   Contextual menus are already shortcuts to task-specific commands;
>>   it's redundant to display keyboard shortcuts too.
>
> I'm not sure I agree with those guidelines -- displaying the key
> bindings increases the discoverability of those key bindings.

For example, how to explain to the users why a keybinding
is displayed for "Undo", but not for "Redo".

Another question is why currently `C-x u' is displayed
instead of the shorter `C-/' for "Undo".

So maybe simpler just to disable keys only for "Undo"
with ‘:keys ""’.  Or the other way around: to manually
add some suggested keybinding to "Redo".