Add support for class visibility

[Matthias Schoettle]

To fully facilitate reuse, classes need a visibility in order to be able to know whether they can be used in the reusing model. Also, this will provide the usage interface.

The following describes what needs to be done, as discussed in the meeting on 05.03.2015:

• Add a visibility enum (ClassifierVisibility) to Classifier in the metamodel with the following literals: Public and Package (which is aspect-package). The default is Package.
• In the GUI, a visibility field should be added to the ClassifierView (on the left of the class name).
• When a user creates an operation (or changes the visibility) that is public, the class visibility needs to be changed to public (if it is not already). This needs to be done through the controller (commands).
• Finally, the weaver needs to be updated to
  • decrease the visibility to package for depends (reused concern)
  • keeps the visibility in case of extends instantiations