Ignore modifier mouse actions when the pointer is constrained

Summary:
If the pointer is constrained all mouse events should go to the window.
Also our Alt+click. To use Alt+click nevertheless one can just unconfine
the window.

CCBUG: 399375

Test Plan: Run the adjusted autotest before and after change

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15982