[effects/presentwindows] Allow closing windows on middle-click

Summary:
Plasma's Task manager exposes an optional  feature whereby the user
can middle-click on a window to close it, but the Present Windows effect
does not do the same.

The presence of a close button you can left-click does not replace the desirable
feature to be able to middle-click on a window to close it, because then the
whole window becomes a click target, so it can be much much faster than
having to aim for the little close button. Also it's off by default, so a user
who goes out of their way to turn it on is signaling that they want to accept the
risk of accidentally closing a window by accident.

Finally, the feature is not allowed for left-click, so people can never accidentally
wreck Present Windows for themselves by assigning it to left-click by accident
and then mistakenly closing their windows.

This reverts commit 55585514.

FEATURE: 321190
FIXED-IN: 5.17.0

Test Plan:
Set "Close window" in the Present windows effect, trigger effect, and middle-click on window

{F6815303}

Reviewers: #kwin, davidedmundson, broulik, zzag, #plasma, hein, mart

Reviewed By: #kwin, #plasma, mart

Subscribers: mart, abetts, apol, zzag, luebking, kossebau, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21083