Handling of default accessibility makes code duplication necessary

[mor-n4]

The computation of a type's / member's actual accessibility in cases no accessibility modifier was declared in the source code is done in the types model (i.e. Types.xcore in methods #getTypeAccessModifier() and #getMemberAccessModifier()). This means that without a type model element, e.g. only on the basis of the AST, it is not possible to reuse this logic, forcing developers to duplicate it.

The goal of this user story is to

1. refactor the above computation of default accessibility to make it reusable and
2. adjust all places in the code where this logic has already been copied.
   Search code for GH-2153 to find some places (not complete!).