Feedback about empty lists

The UX is pretty bad for all first-time users of F-Droid, because they get an empty screen and no feedback about what to expect next.

This adds the text shown here to the screen when there is nothing else to show.

It also shows a banner at the top saying "Updating repositories" whenever the UpdateService is doing its thing. This is shown on both the latest + categories screen.

Finally, it fixes up the situation where enabling only the GP repo will show a blank screen on "Latest" due to no apps being updated recently. Instead, we now show the last 200 apps which were updated from any repo.