xkb: Switch delay to a group

[Ilya Murav'jov]

> I've found shift+shift (i.e. left shift + right shift) to be the best
> option. Partly because I've remapped caps lock to be control.
> 
Thanks for the suggestion, but I think it not a good solution to suggest
everyone to learn again what keybinding suits well. Many people use
Ctrl+Shift/Alt+Shift without problems under M$ OS, and you cant tell
them to change their habits.

And I just suggest to implement the option I mentioned earlier. I think
it is not hard to do but I'm not entirely sure... So it would be nice if
someone knowing the XKB code confirm my guess.

Ilya