Scope the BreakingChanges model on changes that may actually have an impact

[tdegueul]

Currently, the BreakingChanges models store everything that has changed between two versions of an API, including elements that may not be accessible from the outside (eg. private fields and methods).

While this information may come handy in some cases, it would be better to restrict the changes we report to those elements only that can be accessed from the outside. I expect there are many cases to take into account and, I believe, a lot of non-trivial ones.

In the case of IoC/Hollywood (cf. #27), for instance, changes in a protected method of a class that is expected to be extended by the client may have an impact. Not so much in the case where the client is just invoking library methods.

[tdegueul]

Btw, this is related to some CM's requirements: i) Identify the public API of a library ii) Identify changes in an API.