Slightly improve discoverability for changing the wallpaper

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

CCBUG: 391485

Test Plan:
Desktop Toolbox menu:
{F5769965}

Context menu:
{F5769966}

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

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

Tags: #plasma

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