Merge: Protected setters

Setters of attributes are private by default. This cause some issue as they are exposed as initializers but still invisible in client modules.
The proposed solution to resolve this inconsistency is to make setters protected by default so they are visible in client modules but only usable in subclasses.

This is a change is the spec, to RFC label is set is people have comments on the sanity/usability/kiss/pola/etc.

Close #1552

Pull-Request: #1567
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>

diff --cc src/modelize/modelize_property.nit
Simple merge

diff --cc tests/sav/nitmetrics_args1.res
Simple merge