On a fresh install, with no downloaded repositories, users were forced
to restart Rigo in order to have the search function fully functional.
If a local repository is configured but not downloaded (thus, not
available) the same bug happens.
It turned out to be Entropy._enabled_repos, returned by Entropy.repositories()
which didn't get re-initialized after a repository update. This commit adds
a _validate_repositories() call inside _repositories_updated_signal().
97aeed94be