introduces CtModel for #584

[monperrus]

Currently, the notion of "model" in Spoon is implicit, and the factory takes this role. However, it's not the way it should be. It's been a long time we wanted to introduce CtModel, #584 triggered it.

The tentative interface is:

/** represents a Java program, modeled by a set of compile-time (Ct) objects
 * where each objects is a program element (for instance, a CtClass represents a class).
 */
public interface CtModel {

    /** returns the root package */
    CtPackage getRootPackage();

    /** returns all types of the model */
    Collection<CtType<?>> getAllTypes();

    /** returns all packages of the model */
    Collection<CtPackage> getAllPackages();

    /** process this model with the given processor */
    void processWith(Processor<?> processor);

}

The workflow of #584 is tested in FactoryTest.testIncrementalModel.

This has triggered cleaning the factory infrastructure.

One more commit is a utility constructor in AbstractFilter.

Done in AF145.

[GerardPaligot]

Unify implementation

[GerardPaligot]

@leventov What do you think about this for your issue #584?