Function add_mail 

fn add_mail(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    address: String,
    host: String,
    port: u16,
    security: Security,
    login: String,
    password: Option<String>,
)

A mail account added: tested with its password (given, or, None, the one its address’s calendars and contacts use), then kept; its watcher started.