Summary:
A client might delete its pointer lock/confinement object. This is supposed to
directly remove the pointer lock/confinement in KWin, but did not explicitly
until now.

BUG: 388885

Test Plan:
Tested manually with Neverball, Nexuiz and the new pointer constraints test
application. The pointer constraints autotest is also appended.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, graesslin, kwin

Tags: #kwin

Maniphest Tasks: T8923

Differential Revision: https://phabricator.kde.org/D13466