Our handling of left vs. right modifiers keys (shift, ctrl, etc.) was
largely not to spec. This patch adds explicit UIEvents::KeyCode values
for these keys, and updates the UI to match native key events to these
keys (as best as we are able).
On Linux/Windows, the ctrl key is used in conjunction with arrow keys to
jump word-by-word in text documents. On macOS, the option key is used
(which is mapped to the alt key code).
