Add QCursor::setPos() on Wayland

Support for cursor constraints is available in all mayor Wayland compositors nowadays: https://wayland.app/protocols/pointer-constraints-unstable-v1#compositor-support

It would be great if that could be added to Qt. There is even documentation on how the API will be named, once it finally makes it into stable.

While the feature is considered "unstable" and "experimental", it's age and it being implemented in all mayor compositors suggests that it has a certain quality.