Add option to make the transparent window blurry

[rhysd]

This PR adds blur option to the iced::window::Settings struct. When enabling it, the window has a blurry effect. The option value will be passed to winit.

Here is a screenshot of example:

use iced::theme::Style;
use iced::widget::{container, text};
use iced::window::Settings;
use iced::{Color, Element, Length::Fill, Theme};

pub fn main() -> iced::Result {
    iced::application("Test", Test::update, Test::view)
        .window(Settings {
            transparent: true,
            blur: true,
            ..Default::default()
        })
        .style(Test::style)
        .run()
}

#[derive(Default)]
struct Test;

impl Test {
    fn update(&mut self, _: ()) {}

    fn view(&self) -> impl Into<Element<()>> {
        container(text("This is test")).center(Fill).style(|_| {
            container::Style::default()
                .background(Color::from_rgba(0.0, 0.0, 0.0, 0.3))
        })
    }

    fn style(&self, theme: &Theme) -> Style {
        Style {
            background_color: Color::TRANSPARENT,
            text_color: theme.palette().text,
        }
    }
}

(To make this example work on macOS, the change for #2727 is also needed.)

[rhysd]

I haven't explained the use case of this feature.

My use case for this feature is creating a visual effect for macOS apps. It is common for the side menu of macOS apps to have a blurry appearance. For example, Finder (the default file manager in macOS) has a similar effect, as shown below. I would like to achieve the same effect for my macOS app to provide a natural look and feel.