Implement support for the wp_pointer_constraints protocol

The wp_pointer_constraints protocol is a protocol which enables clients
to manipulate the behavior of the pointer cursor associated with a seat.

Currently available constraints are locking the pointer to a static
position, and confining the pointer to a given region.

Currently locking is fully implemented, and confining is implemented for
rectangular confinement regions.

What else is lacking is less troublesome semantics for enabling the lock
or confinement; currently the only requirement implemented is that the
window that appears focused is the one that may aquire the lock.

This means that a pointer could be 'stolen' by creating a new window that
receives active focus, or when using focus-follows-mouse, a pointer
passes a window that has requested a lock. This semantics can be changed
and the protocol itself allows any semantics as seems fit.

https://bugzilla.gnome.org/show_bug.cgi?id=744104