Changed Cocoa activation to trigger on WillAppear instead of DidAppear.

My original thought was that activation/deactivation should trigger off of the `Will` events instead of the `Did`. However, I think the deactivation makes sense to keep on the `Did` since you don't technically want to deactivate a component that may be animating offscreen - It has the potential to look weird if deactivation is tied to some visual component.