Add whenReady to complement isReady by returning a Promise

[ElliotNB]

Today, we have the isReady method, which returns true when the component (and all child components) are no longer processing any UI updates, data requests, etc. We should add the method whenReady which returns a Promise that resolves whenever the component becomes ready. If the component is already ready at the time whenReady is invoked, then the Promise should immediately resolve.

We've encountered a few different scenarios where it's beneficial to have a listener for a component is completely finished loading, rendering, etc.

@atorres-t4m might be interested in tackling this one.