Previously we identified keys in keyboard events by the 'key' property
if it was set, and 'keyCode' otherwise.

This turns out to be problematic as Firefox no longer leaves 'key'
undefined (so we fall back to using 'keyCode'), but instead sets 'key'
to 'MozPrintableKey' for all printable keys.

This meant that when (printable) keys are released, we can't match it
against the corresponding keydown event, and instead just send a keyup
event for the last keydown received.

Now, if both 'key' and 'keyCode' are set, use the concatenation of both.
Otherwise prefer 'keyCode', as that is at least unique for every key.

This should let us release the right keys on keyup events.