Provide alternatives for Show Desktop and Minimize All Windows widgets

Summary: Provide alternatives, so you can quickly switch between the two widgets.

Test Plan: Switch to an alternative to the Show Desktop or Minimize All Windows widget.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: zzag, davidedmundson, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20895