Enable removing multiple devices at once

Summary:
With the device list only allowing one item to be selected, removing
many devices takes a lot of clicking. Allowing multiple selection
solves that with a familiar UI pattern.

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24110