Cancel popup if the user clicked window decoration of parent window

Summary:
So far the window decoration was not considered and e.g. right clicking
the window decoration resulted in two open popups - one by KWin and one
by the application. This change addresses the problem by ensuring the
popup gets cancelled if the decoration is clicked. It's considered not
being part of the window.

Test Plan: Added test case which fails without the change

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5388