Function rename_category 

pub(crate) fn rename_category(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    from: &str,
    to: &str,
) -> Result<(), String>

A category renamed on every task that has it, or taken off them all (to empty); a name already there merges with it. Written at once, then the accounts send the changes.