Use same intro page system settings icon as in the desktop file
Summary: Use the same system settings icon in the intro page that is used in the desktop file for consistency and compatibilty with other icon themes.
Test Plan: Open the system settings (maybe also with a different icon theme).
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, VDG, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18561