pk: Notify of a new distro upgrade from AppStream

PackageKitNotifier already had the logic to notify of a distro upgrade from PackageKit::Daemon::getDistroUpgrades(). However, not all pk backends implement that that.

In the PackageKitBackend we already implemented upgrading using versions from AppStream, so check the same metadata in the notifier.

I have no evidence that the old code path was actually being used by anyone: in facts the notification was misconfigured and thus couldn't be shown, on modern KF versions at the least. Nonetheless the old logic remains intact while this is implemented in parallel.

Contrary to the old logic that invokes PackageKit::Daemon::upgradeSystem(version) immediatly without showing any progress indicator, this notification simply launches the discover UI where the already present upgrade logic will guide the user through an informed distro upgrade.