Fix blurry icons by adding UseHighDpiPixmaps flag

Summary: When using fractional scaling, icons on the top and on both sides are blurred. This patch fixes the issue.

Test Plan:
No side effects.

Before:
{F8287055}
After:
{F8287056}

Reviewers: #kde_applications, yurchor

Reviewed By: yurchor

Subscribers: yurchor, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D29450