Change workspace theme category icon

Summary:
Use a proper workspace theme icon for the workspace theme category.
{F6583141}

Test Plan: Open the system settings.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18677