Default navigablity for associations to implementation classes

[Matthias Schoettle]

When creating an association between a class and an implementation class, the association is bi-directional. However, by default, it should be uni-directional (from class to impl class). And the navigability should not be changeable. For instance, currently, only the navigability to the impl class can be changed, not the opposite.

In summary:

• the navigability on creation has to be Class -----> Implementation Class
• the navigability is not changeable

[Romain Alexandre]

References #276: Association between a class and an implementation class is uni-directional (from class to impl class)

→ <<cset dcf7da9de95f>>

[Romain Alexandre]

Resolve #276 : Branch merged with master