Fix blurry icons in titlebar appmenu by adding UseHighDpiPixmaps flag

Summary: When using fractional scaling, icons in titlebar appmenu are blurred in all apps. This patch fixes the issue.

Test Plan: No side effects.

Reviewers: Frameworks, davidedmundson, anthonyfieroni

Reviewed By: anthonyfieroni

Subscribers: ngraham, anthonyfieroni, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29815