Fix issue with low-res image in Quick Open menu

Summary: Fix the 4th issue from https://phabricator.kde.org/T664 ("Quick Open: Pixmaps in list view items blocky")

Test Plan: Tests passed; pixmaps look fine with QT_DEVICE_PIXEL_RATIO=2 and fonts DPI forced to 192.

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: apol, kdevelop-devel, KDevelop

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D848