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