Don't create pixmaps for loglevel icons

Summary: We can pass QIcon in all places. Depends on D26347 to be able to see any icons

Test Plan: Still have icons on the left side of the log list

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: anthonyfieroni, apol

Maniphest Tasks: T12306

Differential Revision: https://phabricator.kde.org/D26350