Fix blurry icons by adding UseHighDpiPixmaps flag

Summary:
When using fractional scaling, icons on the top are blurred. This patch fixes the issue.
The only change needed to the code is to find a better placement for the line.

Test Plan:
No side effects.

Before:
{F8280480}
After:
{F8280479}

Reviewers: #kde_applications, sdepiets

Reviewed By: sdepiets

Subscribers: davidedmundson, sdepiets

Differential Revision: https://phabricator.kde.org/D29376