#11 removing a class from a model creates compilation errors

[Massimiliano Zattera]

Create a model
Add a few classes with their operations and attributes.
Implement the XOCL model.
Remove one of the classes and Implement again.

You should experience an error if you try to run the model since the class
xxxImpl, the class interface, the class provider are not removed and now will
reference some constants for feature IDs that are not defined any longer.
Moreover, if you remove all of these classes, the Factory will still contain an
import directive to import some of them, which needs manual removing of the
include directive.

This should be handled automatically and .java files should be removed when
their model counterparts are removed or, at least, user should be prompted for
doing so.