[kcmkwin/kwintabbox] Use a more appropriate icon

Summary:
The current icon (window-duplicate) is semantically incorrect and visually disconnected from what it actually does.

This one is much more appropriate on both counts.

Test Plan:

Reviewers: VDG, KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20801