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/collab_ui/src/collab_ui.rs

@@ -13,8 +13,8 @@ use call::{report_call_event_for_room, ActiveCall};
 pub use collab_panel::CollabPanel;
 pub use collab_titlebar_item::CollabTitlebarItem;
 use gpui::{
-    actions, point, AppContext, DevicePixels, Pixels, PlatformDisplay, Size, Task, WindowContext,
-    WindowKind, WindowOptions,
+    actions, point, AppContext, DevicePixels, Pixels, PlatformDisplay, Size, Task,
+    WindowBackgroundAppearance, WindowContext, WindowKind, WindowOptions,
 };
 use panel_settings::MessageEditorSettings;
 pub use panel_settings::{
@@ -121,5 +121,6 @@ fn notification_window_options(
         is_movable: false,
         display_id: Some(screen.id()),
         fullscreen: false,
+        window_background: WindowBackgroundAppearance::default(),
     }
 }