Add the Show Desktop applet (moved from kdeplasma-addons)
Summary: Move the Show Desktop applet from kdeplasma-addons to plasma-desktop, so it can be added to the panel by default later.
Test Plan: The Show Desktop applet should now appear in the "Add Widgets..." dialog when kdeplasma-addons is not installed.
Reviewers: Plasma, VDG, ngraham, apol
Reviewed By: VDG, ngraham, apol
Subscribers: apol, ltoscano, ngraham, VDG, plasma-devel, Plasma
Tags: Plasma
Maniphest Tasks: T7917
Differential Revision: https://phabricator.kde.org/D20818