Automatically update apps list

[nigeldeakin]

If the top level "all apps" dashboard is being displayed, and a new app is then created (or an existing app is then deleted) using the CLI (or perhaps another UI window) then the list of apps remains unchanged. The list of apps is not updated until the page is refreshed.

The page should automatically detect that a new app has been created (or deleted) and update the display accordingly. We already have a background thread fetching stats, so it is not unreasonable to also have a background thread fetching a list of apps.

[nigeldeakin]

See also #42, which describes the same issue for the routes list for a specific application.