Stronger encapsulation of collection fields in MavenProject with im…

…mutability and defensive copies.

The following rules are applied in this commit (the previous situation was a mix of different practices):

* All getter methods except `getDependencyArtifacts()` return an empty collection instead of null.
* All setter methods except delegates make a defensive copy of the given collection, preserving order.
* All getter methods returns an unmodifiable (but not necessarily immutable) collection.
* The collections of properties that do not have an `addFoo(…)` method are immutable.
* The `clone()` method copies all non-immutable collections, and only them.
* `MavenProject(MavenProject)` assigns fields directly without invoking setter methods.