Sort update items by completion

Summary:
Sort update items by completion.

BUG: 390911

Test Plan: Tested locally with dummy and packagekit@alpm (because I already had the rest up to date, needs more testing)

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14250