Slightly improve discoverability for changing the wallpaper

Summary:
Use an icon that looks like it might be related to changing the wallpaper for the Configure Desktop menu item. This slightly improves discoverability for changing the wallpaper.

CCBUG: 391485

Test Plan:
Desktop Toolbox menu:

Context menu:

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: mart, hein, broulik, richardbowen, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11253