[Task Manager] Bring to current desktop with middle-click

Summary:
This adds a new possible action when middle-click is used on a task
in the task manager: bringing that task to the current desktop.

FEATURE: 360250

Test Plan:
In the task manager settings > Behavior > On middle-click, select
'Bring to the current desktop', and make sure 'Show only tasks from
the current desktop' is deselected.
Having several applications opened on different virtual desktops,
middle-click on a task from another desktop and check that it is
brought to the current one.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20773