Use KeyboardEvent.key if available,
match both lower and upper case letters to keep previous behaviour

KeyboardEvent is a mess.

KeyboardEvent.which gives you, in theory, a decimal representation of the key pressed.
"r" or "R" gives you 82, which is "R", you can look at KeyboardEvent.shiftKey,
but you don't have access to capslock...

Maybe you want to use numbers, but of course NumPad will not give you the same than
"normal" numbers ...

Now if you use something else than letter, for exemple "?",
on a QWERTY keyboard "/" and "?" gives you 191,
on a AZERTY keyboard "," and "?" gives you 188, so we have to stick to letters.

This was for keydown and keyup events, keypressed event return the real char
(lower "a", "/", "?", ...) but it fails in some cases

The only non broken property is KeyboardEvent.key,
but it's only supported since Chrome 51, Opera 38, and not supported by Safari
https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key

A good reference & test:
http://unixpapa.com/js/testkey.html
http://unixpapa.com/js/key.html

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>