Fix blurry icons by adding UseHighDpiPixmaps flag
Summary: When using fractional scaling, icons on the top are blurred. This patch fixes the issue.
Test Plan: No side effects.
Reviewers: KDE Games, yurchor
Reviewed By: yurchor
Subscribers: yurchor, kde-games-devel
Differential Revision: https://phabricator.kde.org/D29654