CDK-451: View isReady / signalReady

This PR builds on https://github.com/kite-sdk/kite/pull/98 and https://github.com/kite-sdk/kite/pull/99 adding support for isReady / signalReady on Views.
Shouldn't this go before the view is signalled? And it doesn't look like it is needed because writing is independent of signalling (unless we wanted to add un-signal when starting work).