Port away from deprecated DesktopIcon
Summary: Use QIcon::fromTheme
Test Plan: Still get the stopwatch icon as thumbnail when thumbnail is loaded
Reviewers: Gwenview, vkrause
Reviewed By: vkrause
Subscribers: vkrause
Tags: Gwenview
Maniphest Tasks: T11654
Differential Revision: https://phabricator.kde.org/D25693