Add private CPLayoutConstraint & al -_constraintType
Code style - simplification
js engine: better & faster handling of constraint replacement

TODO: why exactly is this happening in the worker and not locally ?
Fixed : translateAutoresizingMaskIntoConstraint flag works.

Example in WorkerTest where a « normal » view with an autoresizing mask
is added to a view and its constraints are automagically generated at
layout time.
TODO: test with existing private views in the FMW.