Software update: add progress step for package download.

[oskirby]

During a large update or on a slow network connection the downloading of .deb files can take a long time with no motion of the progress slider bar. There is probably some trickery with the --print-uris option that will produce some more useful feedback as to how many packages need to be downloaded as part of the upgrade.

[skronstein]

This has been implemented.

[oskirby]

It has not been implemented. During the "Upgrading Packages" phase, the slider moves only when a package is extracted, or configured. No motion occurs when the package is downloaded from the server.