Implementation of PointerConstraints protcol

Summary:
The pointer constraints protocol is an unstable protocol and thus
the implementation follows the semantics of unstable protocols.

The protocol allows to create a constraint on the pointer - either a
lock or a confinement on a surface. Those are not activated at once, but
when the compositor actively grants it.

During lock no further pointer motion is emitted, during confinement the
pointer is kept in a certain area.

This implements T4451.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3466