Refactored the modes management.

- no mode is exposed through RPC anymore
- a single module in charge of handling code documents is now exposed
- a new module has been created: a modes and documents manager, acting like a proxy to call specific modes services on a given document
Updated some instructions for setup of the project.

A step was missing about the build of the parsers.
Added a documentation/tutorial for client implementors.

There are still things to put in it, but it already explains more concretely how to use the backend.