Add Virtual Desktops KCM icon

Summary: It's basically an icon of the desktop pager widget, but with a Shade Black background like the Desktop Grid effect. I also added some subtle shadows.

Test Plan:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18490