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.
Reviewed By: VDG, ngraham
Maniphest Tasks: T7917
Differential Revision: https://phabricator.kde.org/D20817