Function make_invoice 

pub(crate) fn make_invoice(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    id: &str,
) -> String

Makes the invoice of a project’s unbilled hours: its record, its hours marked, its payment expected in the project’s budget. Returns {“number”, “html”, “pdf”} for the window to print, or {“error”}.