General HiDPI enablement for Gwenview's importer
Summary:
This is a follow-up patch to 43f6e15237f3, which enabled HiDPI in the UI
of Gwenview's main app.
Note: Thumbnails are still pixelated as this is not yet fixed in the
respective Gwenview component.
Solves issue 2. in D7581#163655.
Test Plan:
Standard icons all over the UI not pixelated anymore for
QT_SCALE_FACTOR=1.5 gwenview_importer .
Reviewers: gladhorn, hetzenecker
Reviewed By: hetzenecker
Subscribers: ngraham
Differential Revision: https://phabricator.kde.org/D8918