[plasma-pa] Add command to switch all applications to the selected device

Summary:
If you have many applications running (e.g. browser tabs) it's very annoying to manually switch them all to another device. With this context menu option, it's now very easy.

Screenshot:

Reviewers: Plasma, broulik, drosca, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: filipf, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19014