Support cursor hints on locked pointer

Summary:
Implement support for locked pointer cursor hints. Cursor hints can be
retrieved by the compositor either continuously by connecting to the
cursorHintChanged signal or only when needed. In the later case the
compositor must connect to the aboutToUnbound signal of the locked pointer
interface in order to fetch the last hint before the interface is unbound.

Test Plan: Autotest added.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T4692

Differential Revision: https://phabricator.kde.org/D14175