clients: Replace 'confine' client to showcase pointer constraints in general

The 'confine' client is used to showcase the behavior of pointer confinement
through the interface zwp_confined_pointer_v1.

Since zwp_confined_pointer_v1 is part of pointer constraints in general, which
includes pointer locking, it makes sense to augment the scope of the client so
it can serve as a showcase for this category of interfaces through
zwp_pointer_constraints_v1.

Currently 'confine' relies on, and is designed around the limitations of, the
toytoolkit. Adapting the toytoolkit for the new requirements proved
unproductive, especially since we wish to add support for pointer constraints in
sub-surfaces. Hence the solution adopted was to write a new client that would
replace the previous one.

This patch introduces the new client, 'constraints.c', replacing 'confine.c'.

Signed-off-by: Sergio Gómez <sergio.g.delreal@gmail.com>