Make keyboard focus a pointer constraints necessity

Summary:
This patch changes KWin's pointer constraining behavior by only allowing
constraints if the surface has keyboard focus. In case the client activation
state changes, it rechecks it.

Test Plan:
Manually with the pointer constraints test application and opening the
launcher by pressing meta. Also amended autotest.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, davidedmundson, kwin

Tags: #kwin

Maniphest Tasks: T8923

Differential Revision: https://phabricator.kde.org/D13492