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.
Updated the documentation for client implementors.

More examples, some corresctions, ...