Model: Add Clear() method to empty out existing depot info.
HaikuDepot: Move repo fetch to background worker thread.

- Factor out functions for refreshing the repositories and
  retrieving the package lists.

- Add a background worker thread which handles those tasks and
  then notifies the window when the model is ready.

- Check if we already have a repository cache, and if so, skip the
  refresh step, unless forced. The latter will eventually be possible
  via the UI.