Merge: neo_doxygen: Introduce a tool to import a Doxygen model.

Merge: neo_doxygen: Introduce a tool to import a Doxygen model.

This tool converts the XML output generated by Doxygen to a Neo4j graph suitable for `neo.nit`.

In summary, the following is imported:
* Namespaces
* Files, generating one module for each file-namespace pair
* Classes, with inheritances
* Members, with signatures and redefinition relationships (when Doxygen provide them)
* Documentation (without formatting for the moment)

Concerning language-specific support, only some Java keywords are correctly interpreted at this point.

Note: `/contrib/neo_doxygen/gen-all.sh` and `/contrib/neo_doxygen/gen-once.sh` call `nx`, so they require PR #744.

Note2: the merge commit updates `neo.nit` to correctly merge with #872

Pull-Request: #867
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>