Show progress for PackageKit update fetching

Summary: It takes longtime on some distros, so we better show some information about how it's evolving.

Test Plan: Tested on KDE Neon, which is where I've suffered this most

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23324