Read-only constraints

[QuentinRoy]

Protecting an internal constraint is I guess quite easy: one only have to provide a 'public' constraint on its 'private' constraint instead of the 'private' constraint itself. But the setter of this public constraint is still callable and one may still re-set it.

At the opposite, a view on the 'private' constraint would be only get-able. The idea is that it reacts and can be used exactly as any other constraint. It would of course still be invalidated by its parent constraint... But one cannot change directly its value.

Thus, one could expose a bunch of protected constraints as an interface.