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