chore(lib): re-organizing the module

[jeertmans]

This PR refactors a lot of the code in order to have a better organized codebase. If will that I needed to create a second level of submodules, to better distinguish the different parts of this project.

TODO list:

• Document all function and attributes (even those that are not part of the public API);
• Update tests to use new paths (and maybe improve a bit the coverage;
• Try to better re-use code between different guis