GUAC-862: Add keysym to all KeyEvents. Move keysym determination logic to KeydownEvent/KeyupEvent/KeypressEvent.
GUAC-862: Add reliability flag to KeyEvent.
GUAC-862: Only handle keydown directly if reliable. Stop waiting for keypress and handle unreliable keydown if a keyup has already occurred (keypress is impossible).
GUAC-862: Remove dependency on timing.