The KeyboardEvent.keyIdentifier and .keyCode attributes
have been deprecated. the .keyIdentifier attribute has
been replaced by the .key attribute.
Change keyDownHandler to use non-deprecated attributes
KeyboardEvent.code and KeyboardEvent.key.
Change getKeys to use these attributes to identify
which key was pressed.
keyList argument passed to getKeys can contain both
key codes, e.g. "KeyM" and "Space" and key values,
e.g. "m" and " ".
Ilixa have developed this code as a part of the project psychopy/psychopy
but it is now being moved to its own space, with its own issues list and
a more permissive license
