Summary:
The Windows 10 Task Manager equivalent opens a new instance when middle-clicking an
open Task, and when middle-clicking on a thumbnail in the pop-up, that task is closed.

These are some nice shortcuts. This patch implements the same behaviors for our Task
Manager.

Test Plan:
1. Delete `~/.config/plasma-org.kde.plasma.desktop-appletsrc` and restart plasmashell
2. Open an app
3. Middle-click on the app's Task -> see that a new instance is created
4. Keep middle-clicking on the app's Task until it becomes grouped
5. Move the mouse over the grouped task
6. Middle-click on a thumbnail in the pop-up -> see how that instance is closed

Reviewers: #plasma, #vdg, ndavis, hein

Reviewed By: #plasma, #vdg, ndavis, hein

Subscribers: ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21627