Function open_window 

pub fn open_window<V: Render>(
    options: WindowOptions,
    cx: &mut App,
    build: impl FnOnce(&mut Window, &mut App) -> Entity<V>,
) -> Result<(AnyWindowHandle, Entity<V>)>

Open a window with a Base Root and return the window and application content. Applications own quit/close actions and confirmation flows. Call init before opening application windows. The builder returns application content, not another Root.

In an async context, call this inside cx.update.