Show appropriate messages when app lists are empty.

NOTE: Don't merge until after the next stable release.

Any comments on the language is welcome, though I know it is the type of thing for which everyone will have a different idea of what should be said... My goal was to first show an informative message, and then a (very brief) troubleshooting message. Don't want to lecture them, don't want to be completely useless.

For the main "available" app list:

No apps in this category.

Try selecting a different category or updating your repositories to get a fresh list of apps.

"Installed" apps list:

No apps installed.

There are apps on your device, but they are not available from F-Droid. This could be because you need to update your repositories, or the repositories genuinely don't have your apps available.

"Can update" app list:

All apps up to date.

Congratulations! All of your apps are up to date (or your repositories are out of date).

"Swaps apps" list:

No apps available to swap.

Either the device you are swapping with didn't offer any apps to swap, or an error occurred while communicating with it.