Re-export image crate when feature image is enabled

[lucatrv]

Considering for instance the following sample code to add an icon:

fn main() -> iced::Result {
    Example::run(Settings {
        window: window::Settings {
            icon: window::icon::from_file_data(
                include_bytes!("icon.png"),
                Some(image::ImageFormat::Png),
            )
            .ok(),
            ..window::Settings::default()
        },
        ..Settings::default()
    })
}

For function iced::window::icon::from_file_data to be available, the image feature needs to be enabled, which adds the image crate as an iced dependency. But then to access image::ImageFormat::Png, the image crate needs also to be added to Cargo.toml as a build dependency. This increases build time if the two versions differ, or may lead to incompatibilities.

IMHO the same should happen for some other notable features such has tokio, async-std, smol, etc.