gpui: Add support for window transparency & blur on macOS (#9610)

This PR adds support for transparent and blurred window backgrounds on
macOS.

Release Notes:

- Added support for transparent and blurred window backgrounds on macOS
([#5040](https://github.com/zed-industries/zed/issues/5040)).
- This requires themes to specify a new `background.appearance` key
("opaque", "transparent" or "blurred") and to include an alpha value in
colors that should be transparent.

<img width="913" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/7547ee2a-e376-4d55-9114-e6fc2f5110bc">
<img width="994" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/b36fbc14-6e4d-4140-9448-69cad803c45a">
<img width="1020" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/d70e2005-54fd-4991-a211-ed484ccf26ef">

---------

Co-authored-by: Luiz Marcondes <luizgustavodevergennes@gmail.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/zed/src/main.rs

@@ -208,12 +208,21 @@ fn main() {
         watch_file_types(fs.clone(), cx);
 
         languages.set_theme(cx.theme().clone());
+
         cx.observe_global::<SettingsStore>({
             let languages = languages.clone();
             let http = http.clone();
             let client = client.clone();
 
             move |cx| {
+                for &mut window in cx.windows().iter_mut() {
+                    let background_appearance = cx.theme().window_background_appearance();
+                    window
+                        .update(cx, |_, cx| {
+                            cx.set_background_appearance(background_appearance)
+                        })
+                        .ok();
+                }
                 languages.set_theme(cx.theme().clone());
                 let new_host = &client::ClientSettings::get_global(cx).server_url;
                 if &http.base_url() != new_host {