Add Show Desktop applet to default panel

Summary:
Closes T7917

FEATURE: 381295
FIXED-IN: 5.16.0

Test Plan: The default panel should now have a Show Desktop applet.

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/D20820