Support loading state on NavList items

[mperrotti]

This change is based on the patterns documented in #590

Sometimes NavList items have to wait for data to render correctly. For example, if the URL is dynamic, or if we want to make sure the linked view can be loaded before letting users click it.

Since NavList uses ActionList under the hood, we may want to make the change in ActionList and open up the NavList API to support a loading state.

Example

[mperrotti]

Filed in wrong repo