Add some missing keys to getEventKey

`getEventKey` was missing a few keys that are correctly handled by Firefox's implementation of the DOM3 keyboard events.

References:
Firefox key list: https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent.key
Key codes: http://www.aspdotnet-suresh.com/2013/09/character-keycodes-javascript-keyboard-keycodes.html