Introducing real DTOs for tree view too
Javadoc and NPE

Added javadoc to method and avoiding NPE,
when node has no children.
Fixes for IT

Comparisons were wrong, as they happened
with Enum on one side (old non-DTOs) and String on other (new DTOs)
but were accepted and treated as Object vs Object and
checks failed.
Merge pull request #74 from sonatype/extract-treeview-model

Extract treeview model