Remove the Show Desktop applet (it's in plasma-desktop now)
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 be missing from kdeplasma-addons.
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, VDG, plasma-devel, Plasma
Tags: Plasma
Maniphest Tasks: T7917
Differential Revision: https://phabricator.kde.org/D20817