From this commit on the source from wich the application was registered
is remembered. We use this information for two things:
 * For applications which wasn't registered via the http-api the remove
   button is hidden.
 * The `ApplicationDiscoveryListener` only removes stale applications
   (those which are not anymore found via discovery) if the were registered
   via the `ApplicationDiscoeryListener`.

closes #328

To easily add new mvc controllers (e.g. from extensions) a new
annotation `@AdminController` is added.