Since Damus iOS is not an immediate-mode UI like android, we would
rather not poll for results. Instead we need a way to register a
callback function that is called when we get new subscription results.
This is also useful on the android side, allowing us to request a new
frame to draw when we have new results, instead of drawing every second.
Signed-off-by: William Casarin <jb55@jb55.com>
