Enable high DPI pixmaps

Summary:
Enable High DPI pixmaps to make icons in dialogs looks nice
if High DPI is enabled.

BUG: 407726

Test Plan: Checked manually in high DPI environment.

Reviewers: hein, cfeck, ngraham

Reviewed By: cfeck, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D25038