Fix keyboard mappings for international keyboard layouts

Key presses by a client with an international keyboard layout will often fail.

I.e. the key AltGr creates an error message "No mapping for key 255". This key is important to produce the @ symbol i.e. on German language keyboard. The currently used e.keyCode differs by browser vendor and is ambiguous for some keys.

This change relies instead on the event.code, which is stable for most common keys and more complete.

I keep the old implementation to rely on e.keyCode as a fallback if the e.code is not available or the mapping for it is missing.