Add new activities and virtual desktops icons

Summary:
This adds new monochrome icons corresponding to the colorful ones. Also deletes the old monochrome preferences-activies icon and updates the symlinks that pointed to it.

Test Plan: Search for activities and virtual-desktops in Cuttlefish.

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: anthonyfieroni, ngraham, ndavis, kde-frameworks-devel, VDG

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22155