Close the window selected in the task group tooltip on middle click

Summary: In Windows, it is possible to close windows by middle-clicking their previews in the taskbar. This patch adds this feature to Latte Dock. Now, middle clicking an item in the task group tooltip will close the corresponding window.

Reviewers: Latte Dock, mvourlakos

Reviewed By: Latte Dock, mvourlakos

Subscribers: mvourlakos, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19284