Function dropdown_control 

pub fn dropdown_control<O, V, W>(
    key: &'static str,
    options: O,
    value: V,
    write: W,
) -> Control
where
    O: Fn(&App) -> Vec<(SharedString, SharedString)> + 'static,
    V: Fn(&App) -> String + 'static,
    W: Fn(String, &mut App) + 'static,

A dropdown control built from (value, label) pairs.