Use locked cursor position hint

Summary:
Listen for cursor position hint changes and set cursor position to the hint
on unlock if a valid position hint was set.

Test Plan: With pointer constraints test app.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Maniphest Tasks: T4693

Differential Revision: https://phabricator.kde.org/D14176