Extract treeview model

Treeview model was deeply interconnected with Maven Indexer classes (and those were even sent over the wire as DTOs). This is a cleanup, where "proper" DTO classes are introduced, and the tree view related models are moved out into indexer-lucene-model module, as they were in plugin so far.

The latter meant that no other plugin client was able to extern and/or use the "tree view" at all (like procurement).

This change remedies it, by extracting the tree view model, detaching it from ASF MI, and putting them into model module, allowing Nexus client subsystems to use tree views based on index.