Add a preferences-desktop-effects icon
Summary: Add a preferences-desktop-effects icon for the Desktop Effects KCM.
Test Plan:
At 100%:
Reviewers: VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D18457