GradleProjectTree and GradleMultiProjectDef are now simple, serializable, immutable classes without any convenience methods or caches.

From now on NbGradleMultiProjectDef and NbGradleProjectTree is used within the plugin.
NbGradleMultiProjectDef and NbGradleProjectTree can now parse their simplified counterparts.
Adjusted FetchedProjectModels to return the project tree as well.
Made GenericModelFetcher a little more robust.