[frontend][html] Fixes keyboard support

Keyboard support was very buggy because it relied on `event.keyCode`. The keycode depends on the keyboard layout.

I changed this behaviour to use `event.key` instead. This variable contains the char produced by the keyboard event or the keyname. It's much more reliable.
It doesn't seem to be possible. According to https://github.com/ungerik/pkgreflect:
> Problem: Go reflection does not support enumerating types, variables and functions of packages.
Don't know about constants, but I don't think it's possible. :-(