Function save_project 

pub fn save_project(
    file: &Path,
    id: &str,
    edit: &ProjectEdit,
) -> Result<String, String>

Writes a project’s fields from its form, in place, its comments and routes kept; a new project (id not there yet) goes at the end, and an empty id is made from its title. Returns its id. file is the notes folder’s projects’ file (file_in): a file not there yet is made.