Add support for the Orca key being "sticky"
This follows the XKB setting and its meaning, that separate press on the modifier and a key will result in a combination of those.
Closes #20 (closed)
Note: Ideally the sticky setting would be updated as the Xkb setting changes, but it's a lot trickier to monitor the event, and @sthibaul suggested that Orca doesn't handle Xkb changes very well anyway so that it might not make so much sense to support it for that feature only.