Softwareupdater: reusing packages appears idle

[nephele]

When interupting a large update, e.g through power failure the next run of SoftwareUpdater will reuse doenloads and verify them. On slower disks and older hardware this can take considerable time, yet there is no indication in the UI that something happens.

It would be neat to report for individual items that they are already downloaded and how far the checksumming has gotten for those items.