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