Remove all the notification stuff

That was part of an effort to change how data gets passed around, but
it never worked, wasn't used, and wouldn't have solved any actual
problems. So now it's gone.
Do not set image URL on controller when the new value is nil.

The idea is that it will be nil when the binding is changing, and thus the controller may no longer respond to things under that key path and throw an exception if we try anyway. That would suck. And it seems to work.