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