Mainly fixed the code to make it work again, and cleaned a lot

Documentation updates:

- updated the installation procedure
- removed some backlog items (from the roadmap too) and put them as GitHub issues
- removed parts of the documentation obsolete due to the fixes described below
- cleaned and fixed the documentation in general

Updating:

- fixed the communication with the backend, to use proper modules and methods, with the right workflow
- commented broken code

Cleaning:

- removed old unecessary code
- changed big blocks of comments (for visual emphasis): replaced the repeated slashes by stars instead
- changed constant naming scheme (for RPC): now the type of the contant (key, member name, etc.) is put before, for better visual recognition when they are grouped
- fixed some little formatting quirks
- replaced some unchecked casts with safe casts

Backend:

- fixed handling of exceptions from the backend: the exceptions were received form the backend as normal objects, so the Backend class believed there were never error. Now the response status is checked.
- added a custom exception class for backend's exceptions
- added convenient methods to use the backend's editor module and more specifically execute mode's services
- refactred the code for more granularity, maintainability, etc.

Document:

- fixed the model of a document, now it contains only one attribute, which is the document ID returned by the backend

Outline:

- fixed it: wrong key was used to access root access
- more safety on content providing

Activator:

- the plugin doesn't fail at startup anymore if no backend can be found nor launched (however it might still fail at the Editor component level)

Folding:

- fixed it: it simply uses the latest backend options to get the data it actually uses

Updated the main documentation file.