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