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