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