Listen for signal to update number of available packages

[hackel]

I typically run apt updates from the CLI, and really only use this is a notifier. I also use unattended-upgrades. It would be nice if this extension could listen for the dbus signal emitted when an update completes (org.debian.apt.CacheChanged or org.freedesktop.PackageKit.UpdatesChanged?) and automatically update the number of installable packages (without performing an additional network update of the package lists).