Function token_control 

pub fn token_control<G, S>(
    key: &'static str,
    min_exponent: u32,
    max_exponent: u32,
    value: G,
    set_value: S,
) -> Control
where
    G: Fn(&App) -> usize + 'static,
    S: Fn(usize, &mut App) + 'static,

A power-of-two token slider, for use inside a SettingRow.