Add alternatives icon

Summary:
Add a more appropriate icon for "Alternative Widgets".

Test Plan: Search for "alternatives" in Cuttlefish.

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, ngraham, kde-frameworks-devel, VDG

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20367